move nonstandard gamma() etc. to _GNU_SOURCE only

it's not even provided in the library at the moment, but could easily
be provided with weak aliases if desired.

include/math.h

@@ -345,10 +345,6 @@ long double truncl(long double);
 
 extern int signgam;
 
-double      gamma(double);
-float       gammaf(float);
-long double gammal(long double);
-
 double      j0(double);
 double      j1(double);
 double      jn(int, double);
@@ -367,6 +363,10 @@ void        sincos(double, double*, double*);
 void        sincosf(float, float*, float*);
 void        sincosl(long double, long double*, long double*);
 
+double      gamma(double);
+float       gammaf(float);
+long double gammal(long double);
+
 double      lgamma_r(double, int*);
 float       lgammaf_r(float, int*);
 long double lgammal_r(long double, int*);