Add PGAC_PROG_VARCC_VARFLAGS_OPT autoconf macro.

The new macro allows to test flags for different compilers and to
store them in different CFLAG like variables.  The existing
PGAC_PROG_CC_CFLAGS_OPT and PGAC_PROG_CC_VAR_OPT are changed to be
just wrappers around the new function.

This'll be used by the upcoming LLVM support, to separately detect
capabilities used by clang, when generating bitcode.

Author: Andres Freund
Discussion: https://postgr.es/m/20170901064131.tazjxwus3k2w3ybh@alap3.anarazel.de