Minor tweaks to the build process so that we can build 5.1-current on
4.8-stable:

Must build lib/libc before libpthread. Fix how we do this to be more
consistant with how lists are handled in the file. Also, don't bother
to prebuild libc if we're not building libpthread.

Submitted by: ru@
Reviewed by: bde@ (before ru@ submitted it)