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