make: on-the-fly dependency generation

make: on-the-fly dependency generation

- support on-the-fly dependency file generation with gcc >=3.0
 (the dependency files are generated while compiling the object
 file, eliminating another gcc+sed invocation)

- support for using makedepend -f-  for generating dependencies
 E.g.: make cfg MKDEP="makedepend -f-". In general gcc should be
 preferred if available (use this if you don't have gcc and your
 compiler doesn't generate good deps).