Allow building with libstdc++ instead of libstdc++11 if people want to use the old GCC ABI

.circleci/config.yml

@@ -95,11 +95,6 @@ references:
 
         # Setup conan cache
         echo 'export CONAN_USER_HOME=/build_cache/conan' >> $BASH_ENV
-        if [[ "${CXX}" == *"g++"* ]]; then
-          # Conan uses old gcc ABI by default but we can only build with the cxx11 ABI.
-          conan profile new default --detect
-          conan profile update settings.compiler.libcxx=libstdc++11 default
-        fi
 
         # Setup ccache
         sudo ln -s /usr/bin/ccache /usr/local/bin/$CC

cmake-utils/conan-setup.cmake

@@ -1,22 +1,9 @@
 macro(setup_conan)
     include(cmake-utils/conan.cmake)
 
-    if(MSVC)
     conan_cmake_run(
         CONANFILE conanfile.py
         BUILD missing)
-    else()
-        conan_cmake_run(
-            CONANFILE conanfile.py
-            # We'd like to use "BUILD missing" but that doesn't work because conan sometimes seems to download prebuilt packages with compiler.libcxx=libstdc++ even though we specify compiler.libcxx=libstdc++11.
-            # see https://github.com/cryfs/cryfs/issues/336 and https://github.com/conan-io/conan/issues/7264
-            BUILD all
-            SETTINGS compiler.libcxx=libstdc++11)
-    endif()
-    conan_basic_setup(TARGETS SKIP_STD)
 
-    if(CONAN_SETTINGS_COMPILER_LIBCXX STREQUAL "libstdc++")
+    conan_basic_setup(TARGETS SKIP_STD)
-        # TODO Test this warning works correctly and that the proposed solution in the warning message works.
-        message(FATAL_ERROR "Conan is set up to build against libstdc++ (i.e. the legacy GCC ABI). We only support libstdc++11 (i.e. the new GCC ABI).\nPlease add the '-s compiler.libcxx=libstdc++11' argument when running 'conan install'.")
-    endif()
 endmacro()