multiterm: Fix building with recent Vala versions

Vala doesn't allow dynamic code as global variable initializers
anymore, so move the dynamic initialization inside plugin_init().

Interestingly, if this actually used to work is only because the
initialization wasn't actually dynamic in C, as a NULL GList is a
valid one and that's what it was initialized to.
multiterm: Fix vapidir under Automake >= 1.11.0a with VPATH builds

Automake 1.11.0a introduced an incompatible change, that is it now
changes directory to `$(srcdir)` before running `valac`.

This means `$(top_srcdir)`-based paths are most likely to be incorrect
when building out-of-tree (VPATH) if the user used relative path as
srcdir (e.g. invoked configure with something like `../configure`).

To fix that, use the absolute version of `$(top_srcdir)`.
Revert "autotools: Enable subdir-objects as many plugins use sub-directory objects"

This reverts commit 8873b9082c1c98b4f3e97062f0c6f26c9d980e95.

Unfortunately Automake has a bug when using `subdir-objects` and a
object is in a sub-directory that contains rules for creating other
objects.  See http://debbugs.gnu.org/cgi/bugreport.cgi?bug=17354

We use it in GeanyPrj's and GeanyVC's unit tests.

There are other possible fixes while still using `subdir-objects`:
* include the C file directly from the unit test main source file,
  like #include "../src/utils.c";
* switch these plugins to non-recursive Automake setup, so all rules
  are in the same Makefile.am.
Merge branch 'build-fixes'

This fixes all build issues and distcheck.