CMake: PCH doesn't pick up -DCMAKE_CXX_FLAGS=-D_GLIBCXX_USE_CXX11_ABI=0

With GCC 5.2 with the new ABI, using:

cmake -Drpath=ON \
-Dexplicitlink=ON \
-Dxrootd=OFF \
-D_builtin-freetype=OFF \
-DCMAKE_CXX_COMPILER=/usr/local/products/gcc/5.2.0-nogold/bin/g++ \
-DCMAKE_C_COMPILER=/usr/local/products/gcc/5.2.0-nogold/bin/gcc \
-DCMAKE_INSTALL_PREFIX=my_directory \
-DCMAKE_CXX_FLAGS=-D_GLIBCXX_USE_CXX11_ABI=0 \
my_source_directory

error: definition of macro '_GLIBCXX_USE_CXX11_ABI' differs between the
precompiled header ('1') and the command line ('0')

which says that the PCH was built without _GLIBCXX_USE_CXX11_ABI=0

Could you have a look, please?

Cheers, Axel.