wscript: check if tgmath actually compiles, should fix building under freebsd

wscript

@@ -233,7 +233,10 @@ def configure(conf):
 
 	# check if we can use C99 tgmath
 	if conf.check_cc(header_name='tgmath.h', mandatory=False):
-		conf.define('HAVE_TGMATH_H', 1)
+		if conf.check_cc(fragment='''#include<tgmath.h>
+				int main(void){ return (int)sin(2.0f); }''',
+			msg='Checking if tgmath.h is usable', mandatory=False):
+			conf.define('HAVE_TGMATH_H', 1)
 
 	conf.env.DEDICATED     = conf.options.DEDICATED
 	# we don't need game launcher on dedicated