Improve coverage of cpluspluscheck.

Improve coverage of cpluspluscheck.

Formerly, cpluspluscheck was only meant to examine headers that
we thought of as exported --- but its notion of what we export
was well behind the times.  Let's just make it check *all* .h
files, except for a well-defined blacklist, instead.

While at it, improve its ability to use a C++ compiler other than g++,
by scraping the CXX setting from Makefile.global and making it possible
to override the warning options used (per suggestion from Andres Freund).

Discussion: https://postgr.es/m/b517ec3918d645eb950505eac8dd434e@gaz-is.ru