Makefile can target diff. versions of valac/libvala: Closes bgo#724249

VALAC and LIBVALA environment variables respected by Makefile.
Stop builds against gedit > 3.10: Refs bgo#724173

gedit 3.11/3.12 is unsupported at the moment.
Updates for Valencia 0.5.1
Updated to Valencia 0.6.0
Configure blacklisted VAPIs

Blacklisted VAPIs aren't scanned when Valencia first parses symbols,
meaning specific VAPI files can be skipped outright.  This is useful
when multiple VAPIs hold the same or similar symbols (such as gtk+-2.0
and gtk+-3.0, or gee-0.8 and gee-1.0) but a project is only using one
of them.
Update .doap file
Manually wipe Valencia's loaded symbols

Because Valencia can't detect when a file has been changed outside of
gedit or renamed, it's possible for its symbol table to get out of
sync with the actual file contents.  A better solution would be to
detect these changes and reload, but until that occurs, this menu item
allows for all symbols to be wiped from memory.  The next time the
user does a symbol lookup, Valencia will reload all symbols (much as
if the user had quit and restart gedit).
Update to 0.8.0
Fix binding error exposed by Vala 0.25.2
Merge branch 'master' into wip/gedit-3.12