math.c: check argument to tgamma

* math.c (math_gamma): check the argument before calling math
  function `tgamma` for edge cases.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58697 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

configure.in

@@ -1317,7 +1317,6 @@ main()
 		    ac_cv_func___builtin_setjmp=no
 		    ac_cv_func_round=no
 		])
-		ac_cv_func_tgamma=no
 		AC_CHECK_TYPE([NET_LUID], [], [],
 			      [@%:@include <winsock2.h>
 			      @%:@include <iphlpapi.h>])

math.c

@@ -869,7 +869,13 @@ math_gamma(VALUE unused_obj, VALUE x)
     double d;
     d = Get_Double(x);
     /* check for domain error */
-    if (isinf(d) && signbit(d)) domain_error("gamma");
+    if (isinf(d)) {
+	if (signbit(d)) domain_error("gamma");
+	return DBL2NUM(INFINITY);
+    }
+    if (d == 0.0) {
+	return signbit(d) ? DBL2NUM(-INFINITY) : DBL2NUM(INFINITY);
+    }
     if (d == floor(d)) {
 	if (d < 0.0) domain_error("gamma");
 	if (1.0 <= d && d <= (double)NFACT_TABLE) {