Log Message:
When we're substituting a system-provided header file, use the C
preprocessor to find out the absolute path of the system header, so that
we may include it before we add our customizations.
This is especially important when using gcc, due to it's "fixincludes".
For example, <math.h> might not be /usr/include/math.h, but instead
something like /path/to/gcc/lib/gcc/${MACHINE_GNU_PLATFORM}/include/math.h.