Make sure that symbols are declared in math.h iff the appropriate

Make sure that symbols are declared in math.h iff the appropriate
namespaces are visible.  Previously, math.h failed to hide some C99-,
XSI-, and BSD-specific symbols in certain compilation environments.

The referenced PR has a nice listing of the appropriate conditions for
making symbols visible in math.h.  The only non-stylistic difference
between the patch in the PR and this commit is that I superfluously
test for __BSD_VISIBLE in a few places to be more explicit about which
symbols have historically been part of the FreeBSD environment.

PR:		65939
Submitted by:	Stefan Farfeleder <stefan@fafoe.narf.at>