autotools -> meson

Original commit message:

> There are some known regressions regarding local testing setups - since
> everything was kinda half written with the expectation that build dir =
> source dir (which should not be true anymore). But everything builds and
> the test suite runs fine, after several hours spent debugging random
> crashes in libpqxx with MALLOC_PERTURB_...

I have not experienced regressions with local testing.

(cherry picked from commit 4b886d9c45cd2d7fe9b0a8dbc05c7318d46f615d)

.gitignore

@@ -1,48 +1,8 @@
-/.pls_cache
-*.o
 *~
-Makefile
-Makefile.in
-.deps
-.hydra-data
-/config.guess
-/config.log
-/config.status
-/config.sub
-/configure
-/depcomp
-/libtool
-/ltmain.sh
-/autom4te.cache
-/aclocal.m4
-/missing
-/install-sh
+.test_info.*
 /src/sql/hydra-postgresql.sql
 /src/sql/hydra-sqlite.sql
 /src/sql/tmp.sqlite
-/src/hydra-eval-jobs/hydra-eval-jobs
-/src/root/static/bootstrap
-/src/root/static/js/flot
-/tests
-/doc/manual/images
-/doc/manual/manual.html
-/doc/manual/manual.pdf
-/t/.bzr*
-/t/.git*
-/t/.hg*
-/t/nix
-/t/data
-/t/jobs/config.nix
-t/jobs/declarative/project.json
-/inst
-hydra-config.h
-hydra-config.h.in
 result
 result-*
 outputs
-config
-stamp-h1
-src/hydra-evaluator/hydra-evaluator
-src/hydra-queue-runner/hydra-queue-runner
-src/root/static/fontawesome/
-src/root/static/bootstrap*/