Add USE_GITLAB, based on github.mk, to make gitlab MASTER_SITE
handling less repetitive.

To use, set in Makefile:
DISTNAME=    exampleproject-1.2
MASTER_SITES= ${MASTER_SITE_GITLAB:=accountname/}

The following variables alter gitlab.mk behavior:

GITLAB_PROJECT        defaults to PKGBASE
GITLAB_TAG            defaults to PKGVERSION_NOREV
                      sometimes you want to override with v${PKGVERSION_NOREV}
                      SHA-1 commit ids are also acceptable
GITLAB_RELEASE        defaults to not defined, set this to ${DISTNAME}
                      when packaging a release not based on a git tag.
GITLAB_SUBMODULES    manually set submodule information:
                      values should be:
                      GitLab_user GitLab_project tag_or_hash submodule_path
GITLAB_TYPE          overrides the autodetected MASTER_SITE URL scheme:

(nikita)