build: Extract the libbitcoinkernel library

I strongly recommend reviewing with the following git-diff flags:
  --patience --color-moved=dimmed-zebra

Extract out a libbitcoinkernel library linking in all files necessary
for using our consensus engine as-is. Link bitcoin-chainstate against
it.

See previous commit "build: Add example bitcoin-chainstate executable"
for more context.

We explicitly specify -fvisibility=default, which effectively overrides
the effects of --enable-reduced-exports since libbitcoinkernel requires
default symbol visibility

When compiling for mingw-w64, specify -static in both:

- ..._la_CXXFLAGS so that libtool will avoid building two versions of
  each object (one PIC, one non-PIC). We just need the one that is
  suitable for static linking.
- ..._la_LDFLAGS so that libtool will create a static library.

If we don't specify this, then libtool will prefer the non-static PIC
version of the object, which is built with -DDLL_EXPORT -DPIC for
mingw-w64 targets. This can cause symbol resolution problems when we
link this library against an executable that does specify -all-static,
since that will be built without the -DDLL_EXPORT flag.

Unfortunately, this means that for mingw-w64 we can only build a static
version of the library for now. This will be fixed.

However, on other targets, the shared library creation works fine.

-----

Note to users: You need to either specify:

  --enable-experimental-util-chainstate

or,

  --with-experimental-kernel-lib

To build the libbitcionkernel library. See the configure help for more
details.

build shared libbitcoinkernel where we can

configure.ac

@@ -663,6 +663,12 @@ AC_ARG_WITH([libs],
   [build_bitcoin_libs=$withval],
   [build_bitcoin_libs=yes])
 
+AC_ARG_WITH([experimental-kernel-lib],
+  [AS_HELP_STRING([--with-experimental-kernel-lib],
+  [build experimental bitcoinkernel library (default is to build if we're building libraries and the experimental build-chainstate executable)])],
+  [build_experimental_kernel_lib=$withval],
+  [build_experimental_kernel_lib=auto])
+
 AC_ARG_WITH([daemon],
   [AS_HELP_STRING([--with-daemon],
   [build bitcoind daemon (default=yes)])],
@@ -1657,15 +1663,23 @@ AM_CONDITIONAL([BUILD_BITCOIN_UTIL], [test $build_bitcoin_util = "yes"])
 AC_MSG_RESULT($build_bitcoin_util)
 
 AC_MSG_CHECKING([whether to build experimental bitcoin-chainstate])
-AM_CONDITIONAL([BUILD_BITCOIN_CHAINSTATE], [test $build_bitcoin_chainstate = "yes"])
+if test "$build_experimental_kernel_lib" = "no"; then
+AC_MSG_ERROR([experimental bitcoin-chainstate cannot be built without the experimental bitcoinkernel library. Use --with-experimental-kernel-lib]);
+else
+  AM_CONDITIONAL([BUILD_BITCOIN_CHAINSTATE], [test $build_bitcoin_chainstate = "yes"])
+fi
 AC_MSG_RESULT($build_bitcoin_chainstate)
 
 AC_MSG_CHECKING([whether to build libraries])
 AM_CONDITIONAL([BUILD_BITCOIN_LIBS], [test $build_bitcoin_libs = "yes"])
+
 if test "$build_bitcoin_libs" = "yes"; then
   AC_DEFINE([HAVE_CONSENSUS_LIB], [1], [Define this symbol if the consensus lib has been built])
   AC_CONFIG_FILES([libbitcoinconsensus.pc:libbitcoinconsensus.pc.in])
 fi
+
+AM_CONDITIONAL([BUILD_BITCOIN_KERNEL_LIB], [test "$build_experimental_kernel_lib" != "no" && ( test "$build_experimental_kernel_lib" = "yes" || test "$build_bitcoin_chainstate" = "yes" )])
+
 AC_MSG_RESULT($build_bitcoin_libs)
 
 AC_LANG_POP

src/Makefile.am

@@ -29,6 +29,7 @@ LIBBITCOIN_NODE=libbitcoin_node.a
 LIBBITCOIN_COMMON=libbitcoin_common.a
 LIBBITCOIN_CONSENSUS=libbitcoin_consensus.a
 LIBBITCOIN_CLI=libbitcoin_cli.a
+LIBBITCOIN_KERNEL=libbitcoin_kernel.a
 LIBBITCOIN_UTIL=libbitcoin_util.a
 LIBBITCOIN_CRYPTO_BASE=crypto/libbitcoin_crypto_base.la
 LIBBITCOINQT=qt/libbitcoinqt.a
@@ -40,6 +41,9 @@ endif
 if BUILD_BITCOIN_LIBS
 LIBBITCOINCONSENSUS=libbitcoinconsensus.la
 endif
+if BUILD_BITCOIN_KERNEL_LIB
+LIBBITCOINKERNEL=libbitcoinkernel.la
+endif
 if ENABLE_WALLET
 LIBBITCOIN_WALLET=libbitcoin_wallet.a
 LIBBITCOIN_WALLET_TOOL=libbitcoin_wallet_tool.a
@@ -798,8 +802,39 @@ bitcoin_util_LDADD = \
 #
 
 # bitcoin-chainstate binary #
-bitcoin_chainstate_SOURCES = \
-  bitcoin-chainstate.cpp \
+bitcoin_chainstate_SOURCES = bitcoin-chainstate.cpp
+bitcoin_chainstate_CPPFLAGS = $(AM_CPPFLAGS) $(BITCOIN_INCLUDES)
+bitcoin_chainstate_CXXFLAGS = $(AM_CXXFLAGS) $(PIE_FLAGS)
+bitcoin_chainstate_LDFLAGS = $(RELDFLAGS) $(AM_LDFLAGS) $(LIBTOOL_APP_LDFLAGS) $(PTHREAD_FLAGS)
+bitcoin_chainstate_LDADD = $(LIBBITCOINKERNEL)
+#
+
+# bitcoinkernel library #
+if BUILD_BITCOIN_KERNEL_LIB
+lib_LTLIBRARIES += $(LIBBITCOINKERNEL)
+
+libbitcoinkernel_la_LDFLAGS = $(AM_LDFLAGS) -no-undefined $(RELDFLAGS) $(PTHREAD_FLAGS)
+libbitcoinkernel_la_LIBADD = $(LIBBITCOIN_CRYPTO) $(LIBUNIVALUE) $(LIBLEVELDB) $(LIBMEMENV) $(LIBSECP256K1)
+libbitcoinkernel_la_CPPFLAGS = $(AM_CPPFLAGS) -I$(builddir)/obj -I$(srcdir)/secp256k1/include -DBUILD_BITCOIN_INTERNAL $(BOOST_CPPFLAGS) $(LEVELDB_CPPFLAGS) -I$(srcdir)/$(UNIVALUE_INCLUDE_DIR_INT)
+
+# libbitcoinkernel requires default symbol visibility, explicitly specify that
+# here so that things still work even when user configures with
+#   --enable-reduce-exports
+#
+# Note this is a quick hack that will be removed as we incrementally define what
+# to export from the library.
+libbitcoinkernel_la_CXXFLAGS = $(AM_CXXFLAGS) $(PIE_FLAGS) -fvisibility=default
+
+# TODO: For now, Specify -static in both CXXFLAGS and LDFLAGS when building for
+#       windows targets so libtool will only build a static version of this
+#       library. There are unresolved problems when building dll's for mingw-w64
+#       and attempting to statically embed libstdc++, libpthread, etc.
+if TARGET_WINDOWS
+libbitcoinkernel_la_LDFLAGS += -static
+libbitcoinkernel_la_CXXFLAGS += -static
+endif
+
+libbitcoinkernel_la_SOURCES = \
   kernel/bitcoinkernel.cpp \
   arith_uint256.cpp \
   blockfilter.cpp \
@@ -879,19 +914,11 @@ bitcoin_chainstate_SOURCES = \
   validationinterface.cpp \
   versionbits.cpp \
   warnings.cpp
-bitcoin_chainstate_CPPFLAGS = $(AM_CPPFLAGS) $(BITCOIN_INCLUDES)
-bitcoin_chainstate_CXXFLAGS = $(AM_CXXFLAGS) $(PIE_FLAGS)
-bitcoin_chainstate_LDFLAGS = $(RELDFLAGS) $(AM_LDFLAGS) $(LIBTOOL_APP_LDFLAGS) $(PTHREAD_FLAGS)
-bitcoin_chainstate_LDADD = \
-  $(LIBBITCOIN_CRYPTO) \
-  $(LIBUNIVALUE) \
-  $(LIBSECP256K1) \
-  $(LIBLEVELDB) \
-  $(LIBMEMENV)
 
 # Required for obj/build.h to be generated first.
 # More details: https://www.gnu.org/software/automake/manual/html_node/Built-Sources-Example.html
-bitcoin_chainstate-clientversion.$(OBJEXT): obj/build.h
+libbitcoinkernel_la-clientversion.l$(OBJEXT): obj/build.h
+endif # BUILD_BITCOIN_KERNEL_LIB
 #
 
 # bitcoinconsensus library #