config: only define str[n]casecmp if necessary

Recent versions of MinGW-W64 already have these symbols defined and
throw a warning if we attempt to re-define them.

configure.ac

@@ -77,6 +77,7 @@ AC_CHECK_LIB(m, log)
 
 AC_CHECK_HEADERS([linux/kd.h dev/isa/spkrio.h dev/speaker/speaker.h])
 AC_CHECK_FUNCS(mmap ioperm)
+AC_CHECK_DECLS([strcasecmp, strncasecmp], [], [], [[#include <strings.h>]])
 
 # OpenBSD I/O i386 library for I/O port access.
 # (64 bit has the same thing with a different name!)

src/doomtype.h

@@ -21,6 +21,8 @@
 #ifndef __DOOMTYPE__
 #define __DOOMTYPE__
 
+#include "config.h"
+
 #if defined(_MSC_VER) && !defined(__cplusplus)
 #define inline __inline
 #endif
@@ -29,11 +31,15 @@
 // Outside Windows, we use strings.h for str[n]casecmp.
 
 
-#ifdef _WIN32
+#if !HAVE_DECL_STRCASECMP || !HAVE_DECL_STRNCASECMP
 
 #include <string.h>
+#if !HAVE_DECL_STRCASECMP
 #define strcasecmp stricmp
+#endif
+#if !HAVE_DECL_STRNCASECMP
 #define strncasecmp strnicmp
+#endif
 
 #else