Rename "overrides" to "custom."

This helps shift the emphasis from "replacing existing files" to "adding
missing files," without ruling either option out.

project/lib/custom/openal/include/config.h

@@ -0,0 +1,21 @@
+#ifdef HX_MACOS
+
+#include "config-macos-x86_64.h"
+
+#elif defined(HX_WINDOWS)
+
+#ifdef HXCPP_M64
+#include "config-windows-x86_64.h"
+#else
+#include "config-windows-x86.h"
+#endif
+
+#elif defined(HX_LINUX)
+
+#include "config-linux-x86_64.h"
+
+#elif defined (HX_ANDROID)
+
+#include "config-android.h"
+
+#endif