option to regenerate antlr files with system antlr

[slayoo]

As discussed at SF.net:

• https://sourceforge.net/p/gnudatalanguage/patches/39/ with @sfabbro
• https://sourceforge.net/p/gnudatalanguage/patches/100/ with @olebole and @GillesDuvert the idea of shipping all ANTLR files within GDL does not have fans among packagers, for good reasons.

As pointed out in the SF issues, the solution which was introduced e.g. in:

• Gentoo (https://gitweb.gentoo.org/repo/gentoo.git/tree/dev-lang/gdl/files/0.9.5-antlr.patch)
• Debian (https://sources.debian.org/patches/gnudatalanguage/0.9.8-2/Build-with-Debian-s-antlr-instead-of-the-embedded-copy-in.patch/) to link with system antlr has the problem of creating version mismatch between the generated files used in compilation and the system files used.

I believe, we should offer via CMake the option to use system antlr (as in the Gentoo patch) but then trigger a regeneration of all antlr-generated files, i.e.:

$ cd gdl/src
$ for i in *.g; do antlr $i; done;

Another issue is that reportedly the GDL-shipped antlrheaders are modified with respect to the original (https://sourceforge.net/p/gnudatalanguage/patches/100/).

[slayoo]

See also: https://github.com/olebole/gnudatalanguage/pull/4

[slayoo]

For the record, the regeneration is done e.g. in the Gentoo build spec: https://gitweb.gentoo.org/repo/gentoo.git/tree/dev-lang/gdl/gdl-0.9.6-r2.ebuild#n79

[slayoo]

Two points:

• as introduced in #529, the regeneration is now done on Travis and the the output files are checked for compliance with the ones from the repo
• as discussed in #198, there is also the question if to remove at all the ANTLR-generated files and the ANTLR headers from the repository and tarballs?

[GillesDuvert]

Making GDL compile easier for everybody is incompatible with requiring those same people to install yet another dependency---moreover an old version of, ANTLR, no?

[olebole]

Why an old antlr? On Debian, we always use the latest version (currently 2.7.7). Simple patch is here. Up to now, we didn't receive bug reports referring to this.

[GillesDuvert]

I mean, ANTLR has since evolved to ANTLR4. I was not refering to the (excellent!) work of package maintainers but the need to keep the dependencies as small as possible for people that want to recompile GDL by themselves. But I may be mistaken.

[olebole]

Yeam, I just discovered that we also have an antlr3 and an antlr4 package. It seems to be common to include external packages (f.e. astropy does it as well for cfitsio, wcslib and others); and it is OK. It would just be nice if there is a (CMAKE) option that switches to an external installation (and to regenerate the relevant files); this would remove the need to patch the sources.