pgsql: Add C++ support to configure.

Add C++ support to configure.

This is an optional dependency. It'll be used for the upcoming LLVM
based just in time compilation support, which needs to wrap a few LLVM
C++ APIs so they're accessible from C..

For now test for C++ compilers unconditionally, without failing if not
present, to ensure wide buildfarm coverage. If we're bothered by the
additional test times (which are quite short) or verbosity, we can
later make the tests conditional on --with-llvm.

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/6869b4f2584787d9e4cefaab8a4bae1ecbe63766

Modified Files
--------------
config/c-compiler.m4           |  40 ++
configure                      | 905 ++++++++++++++++++++++++++++++++++++++++-
configure.in                   |  50 ++-
doc/src/sgml/installation.sgml |  18 +
src/Makefile.global.in         |  10 +
src/Makefile.shlib             |   1 +
6 files changed, 1013 insertions(+), 11 deletions(-)