Add -MT option

Allows overriding the output file in dependencies, which also allows
outputting those without also outputting the object file.
This, again, mimicks GCC's option.

include/asm/main.h

@@ -35,6 +35,7 @@ extern struct sOptions DefaultOptions;
 extern struct sOptions CurrentOptions;
 
 extern FILE *dependfile;
+extern char *tzTargetFileName;
 
 extern bool oGeneratePhonyDeps;