Unbreak build on glibc 2.5.1

Unbreak build on glibc 2.5.1

Summary:
Also move manually-generated stuff away from folly-config.h and into
Portability.h.  This should be only for things that differs between the
various (compiler, library) pairs used internally at FB; everything else
should be autoconf-ed and therefore go into folly-config.h.

Test Plan: built and ran file_util_test on various platforms

Reviewed By: delong.j@fb.com

FB internal diff: D807067

Blame Revision: D806781