Fix leaking of ICU's c++ interface through libxml on some platforms

This may become unnecessary once the backends are rewritten in c++.
Tested on sabayon linux (which exibited the issue).

src/backend/xml/CMakeLists.txt

@@ -87,7 +87,7 @@ TARGET_INCLUDE_DIRECTORIES (gnc-backend-xml-utils
   PRIVATE ${ZLIB_INCLUDE_DIRS}
 )
 
-TARGET_COMPILE_DEFINITIONS (gnc-backend-xml-utils PRIVATE -DG_LOG_DOMAIN=\"gnc.backend.xml\")
+TARGET_COMPILE_DEFINITIONS (gnc-backend-xml-utils PRIVATE -DG_LOG_DOMAIN=\"gnc.backend.xml\" -DU_SHOW_CPLUSPLUS_API=0)
 
 INSTALL(TARGETS gnc-backend-xml-utils
   LIBRARY DESTINATION lib
@@ -105,7 +105,7 @@ ADD_LIBRARY(gncmod-backend-xml MODULE ${libgncmod_backend_xml_SOURCES})
 TARGET_LINK_LIBRARIES(gncmod-backend-xml gnc-backend-xml-utils gncmod-engine
                         gnc-core-utils ${LIBXML2_LDFLAGS} ${GLIB2_LDFLAGS} ${ZLIB_LIBRARY})
 
-TARGET_COMPILE_DEFINITIONS (gncmod-backend-xml PRIVATE -DG_LOG_DOMAIN=\"gnc.backend.xml\")
+TARGET_COMPILE_DEFINITIONS (gncmod-backend-xml PRIVATE -DG_LOG_DOMAIN=\"gnc.backend.xml\" -DU_SHOW_CPLUSPLUS_API=0)
 
 SET(LIB_DIR lib/gnucash)
 IF (WIN32)

src/backend/xml/Makefile.am

@@ -7,6 +7,7 @@ pkglib_LTLIBRARIES = libgncmod-backend-xml.la
 AM_CPPFLAGS = \
   -I.. -I../.. \
   -DLOCALE_DIR=\""$(datadir)/locale"\" \
+  -DU_SHOW_CPLUSPLUS_API=0 \
   -I${top_srcdir}/src/backend \
   -I${top_srcdir}/src/engine \
   -I${top_srcdir}/src/core-utils \

src/backend/xml/test/CMakeLists.txt

@@ -18,6 +18,7 @@ SET(XML_TEST_LIBS gncmod-engine gnc-qof gncmod-test-engine test-core ${LIBXML2_L
 
 FUNCTION(ADD_XML_TEST _TARGET _SOURCE_FILES)
   GNC_ADD_TEST(${_TARGET} "${_SOURCE_FILES}" XML_TEST_INCLUDE_DIRS XML_TEST_LIBS ${ARGN})
+  TARGET_COMPILE_OPTIONS(${_TARGET} PRIVATE -DU_SHOW_CPLUSPLUS_API=0)
 ENDFUNCTION()
 
 
@@ -74,6 +75,7 @@ GNC_ADD_TEST_WITH_GUILE(test-load-example-account
   XML_TEST_INCLUDE_DIRS XML_TEST_LIBS
   GNC_ACCOUNT_PATH=${CMAKE_SOURCE_DIR}/accounts/C
 )
+TARGET_COMPILE_OPTIONS(test-load-example-account PRIVATE -DU_SHOW_CPLUSPLUS_API=0)
 ADD_XML_TEST(test-string-converters "${test_backend_xml_base_SOURCES};test-string-converters.cpp")
 ADD_XML_TEST(test-xml-account "${test_backend_xml_module_SOURCES};test-xml-account.cpp;test-file-stuff.cpp")
 ADD_XML_TEST(test-xml-commodity "${test_backend_xml_module_SOURCES};test-xml-commodity.cpp;test-file-stuff.cpp")

src/backend/xml/test/Makefile.am

@@ -271,6 +271,7 @@ AM_CPPFLAGS = \
   -I${top_srcdir}/src/engine/test-core \
   -I${top_srcdir}/src/backend/xml \
   -I${top_srcdir}/src/libqof/qof \
+  -DU_SHOW_CPLUSPLUS_API=0 \
   ${LIBXML2_CFLAGS} \
   ${GLIB_CFLAGS} \
   ${GUILE_CFLAGS} \