Work around -Werror,-Wunused-macros with clang-cl

clang-cl as-is does not provide the intrinsics provided by MSVC; you need to
explicitly include Intrin.h (provided by clang) for that.  So, as a hack,
specify CC/CXX as

  clang-cl.exe -FIIntrin.h ...

to have the intrinsics always available.  But Intrin.h includes stdlib.h, so by
the time sal/osl/w32/random.c defines _CRT_RAND_S before including stdlib.h, the
latter has already been included without _CRT_RAND_S support.  So, as a second
hack, specify CC rather as

  clang-cl.exe -D_CRT_RAND_S= -FIIntrin.h ...

But then clang-cl starts to emit -Werror,-Wunused-macros, as defining
_CRT_RAND_S in the main file has no effect here.

Change-Id: I5dfe9872dea7e8eb476d9260f17ab8d8893f48af

sal/osl/w32/random.c

@@ -6,7 +6,10 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/.
  */
+
+#if !defined _CRT_RAND_S
 #define _CRT_RAND_S
+#endif
 
 #include <stdlib.h>
 #include <memory.h>