If needed, this will be done later via machdeps and not preprocessor macros

This reverts commit 1e6f9135.

It also changes pthread_t to match the usual definition on a glibc + Linux.

- used by 8cc among others

The C standard explicitly states that these functions 'convert the initial
portion of the string pointed to by nptr ...'; therefore they do expect
_strings_ (NUL-terminated) as inputs.

Thanks to Mark Dascher for the fix.