Previously we had a mix of uses of AC_CHECK_PROG[S] and AC_PATH_PROG[S].
The only difference between those macros is that the latter emits the
full path to the program it finds, eg "/usr/bin/prove", whereas the
former emits just "prove".  Let's standardize on always emitting the
full path; this is better for documentation of the build, and it might
prevent some types of failures if later build steps are done with
a different PATH setting.

I did not touch the AC_CHECK_PROG[S] calls in ax_pthread.m4 and
ax_prog_perl_modules.m4.  There seems no need to make those diverge from
upstream, since we do not record the programs sought by the former, while
the latter's call to AC_CHECK_PROG(PERL,...) will never be reached.

Discussion: https://postgr.es/m/25937.1501433410@sss.pgh.pa.us