[lumped builds] Only define _CRT_SECURE_NO_WARNINGS if it’s not defined yet.

When glslang is built with some other build system and lumped/unity builds are used,
without the checks this would get “macro is being redefined” warnings/errors.

glslang/MachineIndependent/preprocessor/PpAtom.cpp

@@ -76,7 +76,9 @@ TORT (INCLUDING NEGLIGENCE), STRICT LIABILITY OR OTHERWISE, EVEN IF
 NVIDIA HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 \****************************************************************************/
 
+#ifndef _CRT_SECURE_NO_WARNINGS
 #define _CRT_SECURE_NO_WARNINGS
+#endif
 
 #include <cassert>
 #include <cstdlib>