Added exception to not use the "f" versions of the math functions when

Added exception to not use the "f" versions of the math functions when
not compiling with gcc (gotta love negative logic!).

Apparently, this was exception was in here back in version 1.20, but
with slightly different logic.  When the logic got changed, it seems
that this exception got lost.

Without this exception, compiling with the native solaris compilers
barfs because most of these functions don't exist on solaris.

svn path=/trunk/vorbis/; revision=1306