Fix a few issues with posix.mak

Fixed the following issues:

* mix of target declarations and variable declarations
* wrong filenames for LICENSE and README. Please pay attention when reviewing commits that change filenames.
* File src/rt/mars.h was mentioned but not present
* Variable COPYDIRS defined but not used
* Some targets would attempt to copy files in non-existing directories
* Target copydir incorrectly specified and not necessary
* Target $(DRUNTIME) depends on win32.mak without necessity
* Incorrect dependencies for druntime.zip