build: Suppress warnings from boost and capnproto in multiprocess code

Without this change there are errors from boost like:

/ci_container_base/depends/i686-pc-linux-gnu/include/boost/signals2/expired_slot.hpp:23:28: error: 'what' overrides a member function but is not marked 'override' [-Werror,-Wsuggest-override]
/ci_container_base/depends/i686-pc-linux-gnu/include/boost/signals2/detail/signal_template.hpp:750:32: error: 'lock_pimpl' overrides a member function but is not marked 'override' [-Werror,-Wsuggest-override]
/ci_container_base/depends/i686-pc-linux-gnu/include/boost/signals2/connection.hpp:150:22: error: 'connected' overrides a member function but is not marked 'override' [-Werror,-Wsuggest-override]

There do not seem to be errors from capnproto currently, but add a suppression
for it, too, to be consistent with other libraries.

configure.ac

@@ -1392,6 +1392,9 @@ if test "$with_libmultiprocess" = "yes" || test "$with_libmultiprocess" = "auto"
   PKG_CHECK_MODULES([LIBMULTIPROCESS], [libmultiprocess], [
      libmultiprocess_found=yes;
      libmultiprocess_prefix=`$PKG_CONFIG --variable=prefix libmultiprocess`;
+     if test "$suppress_external_warnings" != "no" ; then
+       LIBMULTIPROCESS_CFLAGS=SUPPRESS_WARNINGS($LIBMULTIPROCESS_CFLAGS)
+     fi
   ], [true])
 elif test "$with_libmultiprocess" != "no"; then
   AC_MSG_ERROR([--with-libmultiprocess=$with_libmultiprocess value is not yes, auto, or no])

src/Makefile.am

@@ -1088,7 +1088,7 @@ libbitcoin_ipc_a_SOURCES = \
   ipc/process.cpp \
   ipc/process.h \
   ipc/protocol.h
-libbitcoin_ipc_a_CPPFLAGS = $(AM_CPPFLAGS) $(BITCOIN_INCLUDES)
+libbitcoin_ipc_a_CPPFLAGS = $(AM_CPPFLAGS) $(BITCOIN_INCLUDES) $(BOOST_CPPFLAGS)
 libbitcoin_ipc_a_CXXFLAGS = $(AM_CXXFLAGS) $(PIE_FLAGS) $(LIBMULTIPROCESS_CFLAGS)
 
 include $(MPGEN_PREFIX)/include/mpgen.mk