MinGW: Fix str*casecmp() macro redef. warning

MinGW defines strcasecmp() and strncasecmp() as macros in string.h if
__CRT__NO_INLINE is defined, which will be the case when including any
of the Win32 API headers.

Closes #594

tjutil.h

@@ -27,9 +27,13 @@
  */
 
 #ifdef _WIN32
+#ifndef strcasecmp
 #define strcasecmp  stricmp
+#endif
+#ifndef strncasecmp
 #define strncasecmp  strnicmp
 #endif
+#endif
 
 #ifdef _MSC_VER
 #define SNPRINTF(str, n, format, ...) \