Enable -Werror for GCC/Clang builds (#786)

* Enable -Werror for GCC/Clang builds

Fixes many of the errors this produces, and disables a handful that
didn't have solutions that were obvious (to me).

* Check for `-W*` flags empirically

* Remove cl_APPLE_fp64_basic_ops support

* Undo NAN conversion fix

* Add comments to warning override flags

* Remove unneeded STRINGIFY definition

* Fix tautological compare issue in basic

* Use ABS_ERROR macro in image tests

* Use fabs for ABS_ERROR macro

* Move ABS_ERROR definition to common header

test_conformance/computeinfo/main.cpp

@@ -96,7 +96,7 @@ typedef struct _extensions extensions_t;
 // first is greater).
 int vercmp(version_t a, version_t b)
 {
-    if (a.major < b.major || a.major == b.major && a.minor < b.minor)
+    if (a.major < b.major || (a.major == b.major && a.minor < b.minor))
     {
         return -1;
     }