Don't define DIST_SUBDIRS

DIST_SUBDIRS should only be defined if SUBDIRS is conditional.
Also, SUBDIRS should always be a subset of DIST_SUBDIRS:

https://www.gnu.org/software/automake/manual/automake.html#Unconfigured-Subdirectories
Ensure various files get included in make dist

Unfortunately these files aren't included automatically by automake.
Files in a _DATA primary aren't automatically distributed by default,
so the dist_ modifier is necessary.

Additionally. Automake only distributes vala-generated .c files. We
need to ship some vala files because the compile fails without them,
and also it's better for downstream to be able to apply patches to
the source vala files rather than the .c by-products.

These files are necessary to have a compileable tarball.
Use top_srcdir variable for parallel build trees

If we're building in a different directory than top_srcdir,
../ won't always reference top_srcdir.
https://www.gnu.org/software/automake/manual/automake.html#index-Parallel-build-trees
(This is also how gitg does it)
Merge pull request #163 from Hnasar/working-make-dist

Fix make dist so the resulting tarball can be compiled