Adjust configure to use "+Olibmerrno" with HP-UX C compiler, if possible.

This is reported to be necessary on some versions of that OS.  In service
of this, cause PGAC_PROG_CC_CFLAGS_OPT to reject switches that result in
compiler warnings, since on yet other versions of that OS, the switch does
nothing except provoke a warning.

Report and patch by Ibrar Ahmed, further tweaking by me.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/44404f394517f89a0bebb1aaace6e64ad79d2817

Modified Files
--------------
config/c-compiler.m4 |    3 ++
configure            |   87 ++++++++++++++++++++++++++++++++++++++++++++++++++
configure.in         |    4 ++
3 files changed, 94 insertions(+), 0 deletions(-)

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