Fixes the build with recent GCCs (at least 4.8+).

commit 16f5d1843b3720729f50376f522550fcfa787a3c
Author: Kylie McClain <somasissounds@gmail.com>
Date:   Wed Nov 25 18:12:06 2015 -0500

    Include `sys/types.h` header to get `mode_t`

    Do not depend on it being included by other system headers.
    It is not included by others on musl-libc, for example.

--- Source/cmStandardIncludes.h
+++ Source/cmStandardIncludes.h
@@ -54,6 +54,8 @@
 
 #if defined( _MSC_VER )
 typedef unsigned short mode_t;
+#else
+# include <sys/types.h>
 #endif
 
 // use this class to shrink the size of symbols in .o files