Cycles: Post-reintegration tweaks to ensure things do compile

This commit contains all the tweaks which were missing in initial patch
re-integration from the standalone Cycles repository.

This commit also contains an utility cmake macro to help linking targets
with different libraries for release/debug builds, the name currently is

  target_link_libraries_decoupled

it gets a target and list of libraries and makes sure debug builds are
using libraries with "_d" suffix.

After all this changes it'll hopefully be easier to interchange patches
between blender and standalone repositories, because they're now quite
identical.

intern/cycles/util/util_opengl.h

@@ -20,6 +20,12 @@
 /* OpenGL header includes, used everywhere we use OpenGL, to deal with
  * platform differences in one central place. */
 
-#include "glew-mx.h"
+#ifdef WITH_GLEW_MX
+#  include "glew-mx.h"
+#else
+#  include <GL/glew.h>
+#  define mxCreateContext() glewInit()
+#  define mxMakeCurrentContext(x) (x)
+#endif
 
 #endif /* __UTIL_OPENGL_H__ */