Safe String support on Windows

Originally reported as https://sourceforge.net/p/cunit/bugs/71/

Compiling CUnit with Visual Studio 2015 will fail as snprintf is now define in the Universal CRT:

Beginning with the UCRT in Visual Studio 2015 and Windows 10, snprintf is no longer identical to _snprintf. The snprintf function behavior is now C99 standard compliant.

• MSDN (https://msdn.microsoft.com/en-us/library/2ts7cx93.aspx)

This one line path fix the problem:

--- a/CUnit.h
+++ b/CUnit.h
@@ -101,7 +101,7 @@
 #  else
 #    define CU_EXPORT
 #  endif
-#  ifdef _MSC_VER
+#  if _MSC_VER < 1900
 #    define snprintf _snprintf
 #  endif
 #else