Automatically detect if libm is needed to support floordiv

Makefile

@@ -21,6 +21,10 @@ KRKMODS  = $(wildcard modules/*.krk modules/*/*.krk modules/*/*/*.krk)
 
 all: ${TARGET} ${MODULES} ${TOOLS} ${GENMODS}
 
+ifneq ($(shell tools/can-floor-without-libm.sh $(CC)),yes)
+  LDLIBS += -lm
+endif
+
 ifeq (,$(findstring mingw,$(CC)))
   CFLAGS  += -pthread
   LDLIBS  += -ldl -lpthread

tools/can-floor-without-libm.sh

@@ -0,0 +1,8 @@
+#!/bin/sh
+($1 -o /dev/null -x c - 2>/dev/null && echo "yes" || echo "no") <<END
+#include <stdio.h>
+#include <stdlib.h>
+int main(int argc, char * argv[]) {
+	return printf("%f", __builtin_floor(strtod(argv[1],NULL)));
+}
+END