* CPP's defined() is UB outside of a #if line
* Use (void)! cast to prevent recent GCCs & glibc from warning
about ignoring the return status from chdir()
In some cases the license-identification header was missing,
so I added that as well. Replaced the remaining headers that
specifically identified "Versions 3.13.7 and higher".
Makefiles and the build system were deliberately excluded.