Fix MinGW target.

Makefile

@@ -93,7 +93,8 @@ src/asm/asmy.y: ${yacc_pre}
 # install instructions instead.
 mingw:
 	$Qenv PATH=/usr/local/mingw32/bin:/bin:/usr/bin:/usr/local/bin \
-		make CC=gcc CFLAGS="-I/usr/local/mingw32/include ${CFLAGS}"
+		make CC=gcc CFLAGS="-I/usr/local/mingw32/include \
+			-D__progname=\\\"\\\" ${CFLAGS}"
 	$Qmv rgbasm rgbasm.exe
 	$Qmv rgblink rgblink.exe
 	$Qmv rgbfix rgbfix.exe

src/extern/err.c

@@ -21,12 +21,14 @@
  * SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
  */
 
-#include <err.h>
 #include <stdio.h>
 #include <stdarg.h>
 #include <stdlib.h>
+#include "extern/err.h"
 
+#ifndef __MINGW32__
 extern char *__progname;
+#endif
 
 void rgbds_vwarn(const char *fmt, va_list ap)
 {