Include -D_GLIBCXX_USE_CXX11_ABI=0 in pkg-config flags

Ensure that any program using libosrm as a dependency builds against
the same libstdc++ ABI. This is needed as Mason dependencies are
still built against the old ABI.

CMakeLists.txt

@@ -514,7 +514,7 @@ if(ENABLE_MASON)
   set(CMAKE_MODULE_LINKER_FLAGS "${CMAKE_MODULE_LINKER_FLAGS} ${LINKER_FLAGS}")
 
   # current mason packages target -D_GLIBCXX_USE_CXX11_ABI=0
-  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -D_GLIBCXX_USE_CXX11_ABI=0")
+  add_dependency_defines(-D_GLIBCXX_USE_CXX11_ABI=0)
 
   # note: we avoid calling find_package(Osmium ...) here to ensure that the
   # expat and bzip2 are used from mason rather than the system