Fixes #2 - Reverted acx_pthread.m4, no longer call autoheader

On ubuntu (autoconf 2.68, automake 1.12.4, the new, non-deprecated autoconf LANG
macros would not work correctly, PTHREAD_LIBS/PTHREAD_CFLAGS were not being set
correctly.

I'm sure that this macro is workable with non-deprecated stuff, but I was not
able to make a workable solution.

I've reverted it to get past the linker errors encountered.