Add preprocessor macros for different platforms

Add preprocessor macros for different platforms

* configure.ac: MK_CONFIGURE shows config.h was generated by configure.
* src/config.ami: Define MK_AMIGAOS.
* src/config.h-vms: Define MK_VMS.
* src/configh.dos: Define MK_DJGPP.
* src/config.h.W32: Define MK_W32 and WINDOWS32.
* src/build_w32.bat: Let WINDOWS32 be defined by config.h.  Remove
unused setting of WIN32.
* src/job.c: Clean up use of WIN32.
* src/main.c: Ditto.
* tests/scripts/features/default_names: Ditto.