Redirect output of musttail attribute test

Compilation output from test for support of musttail attribute leaked to
the console.

m4/fp_musttail.m4

@@ -5,7 +5,7 @@ AC_DEFUN([FP_MUSTTAIL],
 [
     AC_MSG_CHECKING([whether __attribute__((musttail)) is supported])
     echo 'extern int foo(void); int bar(void) { __attribute__((musttail)) return foo(); }' > conftest.c
-    if $CC -c conftest.c -o conftest.o
+    if $CC -c conftest.c -o conftest.o > /dev/null 2>&1
     then
         AC_MSG_RESULT([yes])
         AC_DEFINE(HAS_MUSTTAIL, 1, [Has musttail])