cmake/msvc: Disable C4996 project wide.

MSVC C4996 triggers upon use of functions like fopen, and suggests
use of non-portable Microsoft replacements. Disable it project wide,
and remove the pragmas littered throughout the tree.

Change-Id: I8b890bbfd3cd7224c25350bd28f334facd8b7783

sample_muxer.cc

@@ -29,11 +29,6 @@
 
 #include "sample_muxer_metadata.h"
 
-#ifdef _MSC_VER
-// Disable MSVC warnings that suggest making code non-portable.
-#pragma warning(disable : 4996)
-#endif
-
 namespace {
 
 void Usage() {