suppress DOS djgpp build warning

gcc -c -O -I../include -DDLB -DUSE_TILES -oo/cmd.o ../src/cmd.c
../src/cmd.c: In function `rhack':
../src/cmd.c:1800: warning: case value out of range
../src/cmd.c:1801: warning: case value out of range

include/pcconf.h

@@ -113,6 +113,9 @@
 #define _NAIVE_DOS_REGS
 #include <stdlib.h>
 #include <string.h>	/* Provides prototypes of strncmpi(), etc.     */
+# ifndef M
+#define M(c)		((char) (0x80 | (c)))
+# endif
 #endif
 
 /*