pkg-config: Remove pthread link flag from Cflags

Remove the threads link library variable references from the pkg-config
Cflags: field, removing -lpthread(s) from the compile flags.

"-l*" linker flags should only be part of the Libs: section and should
not be part of the Cflags: section in pkg-config files.

This was first suggested in
https://github.com/google/googletest/pull/2006 and further discussed in
https://github.com/google/googletest/pull/2483 .