static assertions in C++

Commit df1a699e5ba3232f373790b2c9485ddf720c4a70 introduced a
StaticAssertStmt() into a header file, which will fail if a module
written in C++ uses that header file. Currently, that header file is
not widely used, but it's a potential problem if the use of static
assertions expands.

As discussed in
<https://www.postgresql.org/message-id/7775.1492448671@sss.pgh.pa.us>, a
more general solution would be to add specific C++ support for static
assertions in c.h. Here is a patch for that, extracted from my
previously posted C++ patch set, but also a bit reworked from what was
previously posted.

Also attached is a little C++ test file that one can use to test this
out. (Just compiling it should cause a compiler error without the patch
and a static assertion failure with the patch.)

--
Peter Eisentraut http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services