Add helper for setting environment variable values

* Needed for vulkan layer control

util/test/demos/linux/linux_platform.h

@@ -43,5 +43,7 @@
 
 #define MakeDir(dir) mkdir(dir, 0755)
 
+#define SetEnv(name, value) setenv(name, value, 1)
+
 std::string GetExecutableName();
 uint32_t GetPID();

util/test/demos/win32/win32_platform.h

@@ -55,4 +55,6 @@ uint32_t GetPID();
 
 #define WEXITSTATUS(code) code
 
+#define SetEnv(name, value) SetEnvironmentVariableA(name, value)
+
 #define EXECUTABLE_SUFFIX ".exe"