Check every variant of CMAKE_CXX_FLAGS_<CONFIG>

BTW check_cxx_source_compiles() doesn't  use the _<CONFIG> flags

CMakeLists.txt

@@ -299,15 +299,17 @@ set_target_properties(${PROJECT_NAME} PROPERTIES CXX_STANDARD ${CMAKE_CXX_STANDA
 
 # Hide symbols by default
 # Except where it causes problems, and the situation is weird enough that it's not worth investigating further.
-string(FIND "${CMAKE_CXX_FLAGS}" "-D_GLIBCXX_USE_CXX11_ABI=0" found_ancient_abi_flag)
+#
+# We've seen people set _GLIBCXX_USE_CXX11_ABI=0 which forces GCC to use it's pre-C++11 string implementation,
+# which leads to crashes on shared-lib builds. Search every variant of CXX_FLAGS to see if it's set.
+string(FIND "${CMAKE_CXX_FLAGS} ${CMAKE_CXX_FLAGS_DEBUG} ${CMAKE_CXX_FLAGS_RELEASE} ${CMAKE_CXX_FLAGS_RELWITHDEBINFO} ${CMAKE_CXX_FLAGS_MINSIZEREL}"
+       "-D_GLIBCXX_USE_CXX11_ABI=0" found_ancient_abi_flag)
 if(found_ancient_abi_flag GREATER -1)
-    # We've seen people set _GLIBCXX_USE_CXX11_ABI=0 which forces GCC to use it's pre-C++11 string implementation.
-    # This leads to crashes on shared-lib builds.
-    message(STATUS "Cannot deal with hidden symbols when _GLIBCXX_USE_CXX11_ABI=0 is set, making everything visible")
+    message(WARNING "_GLIBCXX_USE_CXX11_ABI=0 is set. Making all symbols visible to prevent weird crashes")
 elseif(CMAKE_CXX_COMPILER_ID STREQUAL "GNU" AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS "5.0")
     # Ancient GCC leads to crashes in shared-lib builds
     # see: https://github.com/awslabs/aws-crt-cpp/pull/675
-    message(STATUS "Your compiler is too old to deal with hidden symbols, making everything visible")
+    message(WARNING "Ancient compiler (${CMAKE_CXX_COMPILER_ID} ${CMAKE_CXX_COMPILER_VERSION}). Making all symbols visible to prevent weird crashes")
 else()
     set_target_properties(${PROJECT_NAME} PROPERTIES CXX_VISIBILITY_PRESET hidden VISIBILITY_INLINES_HIDDEN ON)
 endif()