win64 dislikes warnings-as-errors

It runs into spurious (?) harmless (?) warnings related
to PTW32_LEVEL and PTW32_LEVEL_MAX not being defined
on windows 64-bit platform.

Change-Id: Ib1d1894100057f7d66a8124558b5b0bc78467c08
Reviewed-on: http://review.northscale.com/2514
Reviewed-by: Steve Yen <steve.yen@gmail.com>
Tested-by: Steve Yen <steve.yen@gmail.com>

configure.ac

@@ -8,7 +8,7 @@ AC_CONFIG_MACRO_DIR([m4])
 AC_DISABLE_STATIC
 ac_cv_func_malloc_0_nonnull=yes
 ac_cv_func_realloc_0_nonnull=yes
-PANDORA_CANONICAL_TARGET(less-warnings, warnings-always-on, force-gcc42)
+PANDORA_CANONICAL_TARGET(less-warnings, force-gcc42)
 PANDORA_REQUIRE_PTHREAD
 PANDORA_HAVE_LIBCURL
 PANDORA_HAVE_LIBSQLITE3