Add -MG

This option allows for automatic dependency detection and generation:
as soon as a missing file is found, it is output to the dep file, and
assembly immediately aborts. (No .o file is produced, even if `-o` was
speicified.) This doesn't cause an error, either; the point is that once
the file is added to the dep file, the Makefile is re-parsed, and this
time the file will be generated, so the dep list builds up automatically.
This mimicks GCC's option and behavior.

src/asm/asmy.y

@@ -1004,16 +1004,22 @@ set		: T_LABEL T_POP_SET const
 include		: T_POP_INCLUDE string
 		{
 			fstk_RunInclude($2);
+			if (oFailedOnMissingInclude)
+				YYACCEPT;
 		}
 ;
 
 incbin		: T_POP_INCBIN string
 		{
 			out_BinaryFile($2);
+			if (oFailedOnMissingInclude)
+				YYACCEPT;
 		}
 		| T_POP_INCBIN string comma uconst comma uconst
 		{
 			out_BinaryFileSlice($2, $4, $6);
+			if (oFailedOnMissingInclude)
+				YYACCEPT;
 		}
 ;