build: drop AM_C_PROTOTYPES.

This macro is deprecated as of Automake 1.12 and can no longer be
used. Most likely is not what was intended to be used.

Make sure to explicitly check for string.h after this change as it was
implied by the macro before.

See http://www.flameeyes.eu/autotools-mythbuster/forwardporting/automake.html
for details.
build: look for pthread_create when finding pthread library.

The pthread_mutex_lock function is defined as a stub in libc (for
glibc), so it'll not find the need for -lpthread.

The only reason why flowgrind would link correctly otherwise is that
other libraries bring pthread in, but this check needs to be fixed
when enabling neither pcap nor gsl.
Merge pull request #1 from gentoo/master

Gentoo fix for automake 1.12