add support for pgcc compiler

[rurban]

http://www.pgroup.com/

esp. the darwin hints break it, but also Makefile.SH NAMESPACEFLAGS = -force_flat_namespace

pgcc is very slow though. pgcc -O2 is 40% slower than gcc-6 -O3, pgcc -fast 30% slower
on darwin with the fast haswell-supported apple libc.

[rurban]

Done with 0591a06