PCHs in subdirectories doesn't work without FORCEINCLUDE

[dutow]

At least for GCC, without the FORCEINCLUDE option, the directory where the pch file is generated is added as an include path.

get_filename_component(_name ${_input} NAME)
// ...
set(_pchfile "${_pch_binary_dir}/${_input}")
// ...
list(APPEND _pch_compile_flags -include "${_pchfile}")

This means that if the pch file is in a subdirectory (e.g. "foo/pch.h"), and that directory itself isn't in the include path, the compiler won't be able to find the gch file, and the original header will be used instead.