Drop unintended bitcoin-tx dependency on libevent

Don't include util/url.cpp to libbitcoin_util.a when libevent isn't available.
This fixes a compile error trying to build bitcoin-tx without libevent reported
by Luke Dashjr in https://github.com/bitcoin/bitcoin/issues/18465

Fixes #18465

build_msvc/libbitcoin_util/libbitcoin_util.vcxproj.in

@@ -8,6 +8,7 @@
     <ConfigurationType>StaticLibrary</ConfigurationType>
   </PropertyGroup>
   <ItemGroup>
+    <ClCompile Include="..\..\src\util\url.cpp" />
 @SOURCE_FILES@
   </ItemGroup>
   <Import Project="$(VCTargetsPath)\Microsoft.Cpp.props" />

configure.ac

@@ -1271,7 +1271,7 @@ if test x$use_pkgconfig = xyes; then
         BITCOIN_QT_CHECK([PKG_CHECK_MODULES([QR], [libqrencode], [have_qrencode=yes], [have_qrencode=no])])
       fi
       if test x$build_bitcoin_cli$build_bitcoind$bitcoin_enable_qt$use_tests$use_bench != xnonononono; then
-        PKG_CHECK_MODULES([EVENT], [libevent],, [AC_MSG_ERROR(libevent not found.)])
+        PKG_CHECK_MODULES([EVENT], [libevent], [use_libevent=yes], [AC_MSG_ERROR(libevent not found.)])
         if test x$TARGET_OS != xwindows; then
           PKG_CHECK_MODULES([EVENT_PTHREADS], [libevent_pthreads],, [AC_MSG_ERROR(libevent_pthreads not found.)])
         fi
@@ -1291,7 +1291,7 @@ if test x$use_pkgconfig = xyes; then
 else
 
   if test x$build_bitcoin_cli$build_bitcoind$bitcoin_enable_qt$use_tests$use_bench != xnonononono; then
-    AC_CHECK_HEADER([event2/event.h],, AC_MSG_ERROR(libevent headers missing),)
+    AC_CHECK_HEADER([event2/event.h], [use_libevent=yes], AC_MSG_ERROR(libevent headers missing),)
     AC_CHECK_LIB([event],[main],EVENT_LIBS=-levent,AC_MSG_ERROR(libevent missing))
     if test x$TARGET_OS != xwindows; then
       AC_CHECK_LIB([event_pthreads],[main],EVENT_PTHREADS_LIBS=-levent_pthreads,AC_MSG_ERROR(libevent_pthreads missing))
@@ -1545,6 +1545,7 @@ AM_CONDITIONAL([ENABLE_QT_TESTS],[test x$BUILD_TEST_QT = xyes])
 AM_CONDITIONAL([ENABLE_BENCH],[test x$use_bench = xyes])
 AM_CONDITIONAL([USE_QRCODE], [test x$use_qr = xyes])
 AM_CONDITIONAL([USE_LCOV],[test x$use_lcov = xyes])
+AM_CONDITIONAL([USE_LIBEVENT],[test x$use_libevent = xyes])
 AM_CONDITIONAL([GLIBC_BACK_COMPAT],[test x$use_glibc_compat = xyes])
 AM_CONDITIONAL([HARDEN],[test x$use_hardening = xyes])
 AM_CONDITIONAL([ENABLE_SSE42],[test x$enable_sse42 = xyes])

src/Makefile.am

@@ -523,9 +523,12 @@ libbitcoin_util_a_SOURCES = \
   util/strencodings.cpp \
   util/string.cpp \
   util/time.cpp \
-  util/url.cpp \
   $(BITCOIN_CORE_H)
 
+if USE_LIBEVENT
+libbitcoin_util_a_SOURCES += util/url.cpp
+endif
+
 if GLIBC_BACK_COMPAT
 libbitcoin_util_a_SOURCES += compat/glibc_compat.cpp
 AM_LDFLAGS += $(COMPAT_LDFLAGS)