devel/gprbuild-aux: Upgrade to version GPL 2012

1) Replace custom tarball with Adacore's GPL 2012 release
2) Update license to GPLv3
3) Remove lang/gnat-aux dependency
4) USE_LANGUAGES+= ada (invokes lang/gcc-aux dependency)
5) Update buildlink3.mk to require GPL 2012 version
6) Restore use of pkgsrc wrappers
7) Add Library_Option "-R" support which ignores all automatically
  generated rpaths* which point to the WRKOBJDIR.  DESTDIR isn't
  supported by GPRBuild, so this rpath disabling is necessary to
  avoid references to work directories.
  * gcc and adalib rpaths are excluded with -R option.
8) This Library_Option "-R" matches the same modification added to
  lang/gcc-aux and lang/gnat-aux's project handler.
9) This modification is required to fix www/aws rpath issues which
  appears with recent changes to PKG_DEVELOPER=yes checks.

(marino)