The reason is: starting from GitLab 8.6 this extension becomes hard
dependency of GitLab.

References:

    https://about.gitlab.com/2016/03/22/gitlab-8-6-released/
    -> "Changes for Source installations with PostgreSQL"

    http://www.postgresql.org/docs/current/static/pgtrgm.html

NOTE

There is no way to activate only some extension building at configure
time - it is "all" or "all with all extensions" in postgresql speak (=
"world" make target).

PostgreSQL INSTALL explicitly suggests for selected-extensions install
to jump to appropriate dirs and do `make install` from there.

    http://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=contrib/README;h=5eaeb2451f29877e986f4683c57dd70edde942d5;hb=HEAD#l15

that's why we abuse slapos.recipe.cmmi a bit and do a double

    make install && make -C contrib/pg_trgm/ install