Simplify the C99 conditionals. Mark the drem and gamma functions as
BSD|SVID, the Bessel double functions also XSI and the floats also SUSv3.
signgam is BSD|SVID|XSI, and matherr is SVID. Finally, use the internal
macros to control the symbolic constants.
Signed-off-by: Yaakov Selkowitz <yselkowi@redhat.com>