[pstl] Add a __pstl_config_site header to record the CMake configuration

This commit adds a __pstl_config_site header that contains the value of
macros specified at CMake configuration time. It works similarly to
libc++'s __config_site header, except we always include it as a separate
file instead of concatenating it to the main configuration header.

It is necessary to thread the includes for that header into libc++'s
lit configuration, otherwise we'd be requiring an installation step
prior to running the test suite.

llvm-svn: 368284