updated test for whether CFLAGS was set by the user (#271)

Author: stevengj

m4/ax_cc_maxopt.m4

@@ -26,7 +26,7 @@ AC_REQUIRE([AX_COMPILER_VENDOR])
 AC_REQUIRE([AC_CANONICAL_HOST])
 
 # Try to determine "good" native compiler flags if none specified via CFLAGS
-if test "$ac_test_CFLAGS" != "set"; then
+if test "x$ac_test_CFLAGS" != "xset" -a "x$ac_test_CFLAGS" != "xy"; then
   CFLAGS=""
   case $ax_cv_c_compiler_vendor in
     dec) CFLAGS="-newc -w0 -O5 -ansi_alias -ansi_args -fp_reorder -tune host"