pgsql: Add PGAC_PROG_VARCC_VARFLAGS_OPT autoconf macro.

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/3de04e4ed12d0794e87e1db2e729d126cf183a58

Modified Files
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config/c-compiler.m4 |  48 +++----
configure            | 366 ++++++++++++++++++++++++++++++---------------------
2 files changed, 243 insertions(+), 171 deletions(-)