make signgam a weak alias for an internal symbol

otherwise, the standard C lgamma function will clobber a symbol in the
namespace reserved for the application.

src/math/signgam.c

-#include <math.h>
-int signgam = 0;
+#include "libm.h"
+
+int __signgam = 0;
+
+weak_alias(__signgam, signgam);