configure: Hide error output from --target check

m4/fp_cc_supports_target.m4

@@ -12,7 +12,7 @@ AC_DEFUN([FP_CC_SUPPORTS_TARGET],
    AC_REQUIRE([GHC_LLVM_TARGET_SET_VAR])
    AC_MSG_CHECKING([whether $1 CC supports --target])
    echo 'int main() { return 0; }' > conftest.c
-   if $CC --target=$LlvmTarget -Werror conftest.c > /dev/null ; then
+   if $CC --target=$LlvmTarget -Werror conftest.c >& /dev/null ; then
        CONF_CC_SUPPORTS_TARGET=YES
        AC_MSG_RESULT([yes])
    else