Set CONFIG_OS_SUPPORT correctly in the cmake build.

BUG=https://bugs.chromium.org/p/aomedia/issues/detail?id=76

Change-Id: Iddb07288166af582b52d75e8b2cb37a55e13a196

build/cmake/aom_configure.cmake

@@ -118,6 +118,10 @@ else ()
   endif ()
 endif ()
 
+if (AOM_TARGET_SYSTEM MATCHES "Darwin\|Linux\|Windows")
+  set(CONFIG_OS_SUPPORT 1)
+endif ()
+
 # Test compiler support.
 aom_get_inline("INLINE")