This patch (3rd of 4) contains changes to gcc-interface/Makefile.in and DJGPP own implementation of 
system.ads.

ChangeLog entry:

2016-07-30 Andris Pavenis <andris.pavenis@iki.fi>

* ada/gcc-interface/Makefile.in (LIBGNAT_TARGET_PAIRS): Define for DJGPP target
   (EH_MECHANISM): Define to -gcc for DJGPP
* ada/system-djgpp.ads: New file

Andris