Fix crash when building with _GLIBCXX_USE_CXX11_ABI=0 (#686)

**Issue:**
A user had been building aws-crt-cpp with `CXXFLAGS=-D_GLIBCXX_USE_CXX11_ABI=0`, and that worked fine until the recent change where we started hiding symbols by default (PR #666)

With symbols hidden by default, and `_GLIBCXX_USE_CXX11_ABI=0`, tests would crash in the destructor of `Aws::Crt::String` (which is `std::string` with a custom allocator).

**Description of changes:**
Don't hide symbols when building for the ancient glibcxx ABI.

I don't 100% understand why this fixes the issue, but the situation is esoteric enough that it doesn't seem worth spending more days on this problem.

**Background**
See: https://gcc.gnu.org/onlinedocs/libstdc++/manual/using_dual_abi.html
GCC had to introduce a new ABI for C++11 which removed the copy-on-write optimization for strings, which is forbidden in C++11 and later (new ABI uses a small-string optimization instead). But GCC let users manually set `_GLIBCXX_USE_CXX11_ABI=0` so they could opt back into the old ABI and continue working with libraries compiled for C++03.

I don't think GCC intended devs to continue using this 10 years later, but some people are, because the old copy-on-write optimization has less memory usage.

.github/workflows/ci.yml

@@ -21,7 +21,7 @@ env:
 concurrency:
   group: ${{ github.workflow }}-${{ github.ref }}
   cancel-in-progress: true
-  
+
 jobs:
   linux-compat-use-openssl:
     runs-on: ubuntu-22.04 # latest
@@ -143,6 +143,14 @@ jobs:
         aws s3 cp s3://aws-crt-test-stuff/ci/${{ env.BUILDER_VERSION }}/linux-container-ci.sh ./linux-container-ci.sh && chmod a+x ./linux-container-ci.sh
         ./linux-container-ci.sh ${{ env.BUILDER_VERSION }} aws-crt-${{ env.LINUX_BASE_IMAGE }} build -p ${{ env.PACKAGE_NAME }} --compiler=${{ matrix.compiler }} --cmake-extra=-DBUILD_SHARED_LIBS=ON
 
+  linux-glibcxx-ancient-abi:
+    runs-on: ubuntu-24.04 # latest
+    steps:
+    - name: Build ${{ env.PACKAGE_NAME }}
+      run: |
+        aws s3 cp s3://aws-crt-test-stuff/ci/${{ env.BUILDER_VERSION }}/linux-container-ci.sh ./linux-container-ci.sh && chmod a+x ./linux-container-ci.sh
+        ./linux-container-ci.sh ${{ env.BUILDER_VERSION }} aws-crt-${{ env.LINUX_BASE_IMAGE }} build -p ${{ env.PACKAGE_NAME }} --compiler=gcc-11 --cmake-extra=-DBUILD_SHARED_LIBS=ON --cmake-extra=-DCMAKE_CXX_FLAGS=-D_GLIBCXX_USE_CXX11_ABI=0
+
   linux-openssl-static:
     runs-on: ubuntu-22.04 # latest
     steps:

CMakeLists.txt

@@ -298,9 +298,19 @@ set_target_properties(${PROJECT_NAME} PROPERTIES LINKER_LANGUAGE CXX)
 set_target_properties(${PROJECT_NAME} PROPERTIES CXX_STANDARD ${CMAKE_CXX_STANDARD})
 
 # Hide symbols by default
-# Except for ancient GCC, because it leads to crashes in shared-lib builds
-# see: https://github.com/awslabs/aws-crt-cpp/pull/675
-if(NOT (CMAKE_CXX_COMPILER_ID STREQUAL "GNU" AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS "5.0"))
+# Except where it causes problems, and the situation is weird enough that it's not worth investigating further.
+#
+# 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} ${CMAKE_CXX_FLAGS_MINSIZEREL}"
+       "-D_GLIBCXX_USE_CXX11_ABI=0" found_ancient_abi_flag)
+if(found_ancient_abi_flag GREATER -1)
+    message(WARNING "_GLIBCXX_USE_CXX11_ABI=0 is set. Making all symbols visible to prevent weird crashes")
+elseif(CMAKE_CXX_COMPILER_ID STREQUAL "GNU" AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS "5.0")
+    # Ancient GCC leads to crashes in shared-lib builds
+    # see: https://github.com/awslabs/aws-crt-cpp/pull/675
+    message(WARNING "Ancient compiler (${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()