Clang + libstdc++ binaries to use rpath (#632)

Binaries built with clang + libstdc++ from brew GCC didn't actually use
same libstdc++ at runtime. Instead, they used system provided libstdc++.
This change adds an rpath linker command to instruct ldd to load
libstdc++.so.6 from the same brew GCC library dir that is used during
linking.

Author: nikobockerman

.github/files/CMakeUserPresets.json

@@ -186,8 +186,8 @@
         "PRESET_CXX_FLAGS_LIBSTDCPP_FOR_CLANG_COMMON": "-stdlib++-isystem$penv{HOMEBREW_PREFIX}/opt/gcc@$penv{GCC_MAJOR_VERSION}/include/c++/$penv{GCC_MAJOR_VERSION}"
       },
       "cacheVariables": {
-        "CMAKE_EXE_LINKER_FLAGS": "-stdlib=libstdc++ -L$penv{HOMEBREW_PREFIX}/opt/gcc@$penv{GCC_MAJOR_VERSION}/lib/gcc/$penv{GCC_MAJOR_VERSION}",
-        "CMAKE_SHARED_LINKER_FLAGS": "-stdlib=libstdc++ -L$penv{HOMEBREW_PREFIX}/opt/gcc@$penv{GCC_MAJOR_VERSION}/lib/gcc/$penv{GCC_MAJOR_VERSION}",
+        "CMAKE_EXE_LINKER_FLAGS": "-stdlib=libstdc++ -L$penv{HOMEBREW_PREFIX}/opt/gcc@$penv{GCC_MAJOR_VERSION}/lib/gcc/$penv{GCC_MAJOR_VERSION} -Wl,-rpath,$penv{HOMEBREW_PREFIX}/opt/gcc@$penv{GCC_MAJOR_VERSION}/lib/gcc/$penv{GCC_MAJOR_VERSION}",
+        "CMAKE_SHARED_LINKER_FLAGS": "-stdlib=libstdc++ -L$penv{HOMEBREW_PREFIX}/opt/gcc@$penv{GCC_MAJOR_VERSION}/lib/gcc/$penv{GCC_MAJOR_VERSION} -Wl,-rpath,$penv{HOMEBREW_PREFIX}/opt/gcc@$penv{GCC_MAJOR_VERSION}/lib/gcc/$penv{GCC_MAJOR_VERSION}",
         "CONAN_HOST_PROFILE": "auto-cmake;${sourceDir}/conan-profile-gcc-libstd++-for-clang"
       }
     },