Add BUILD_DIR variable to .ycm_extra_conf.py

[galou]

BUILD_DIR allows to look for the compile_commands.json file
for out-of-source builds, when the build directory is not at the same
level as the source directory.

Originally #7, which was closed because it included changes in untouched lines.

[JDevlieghere]

Hi Gaël,

I've giving this a lot of thought and while I appreciate your PR I'm not convinced it will benefit most of the users of the global YCM config. As explained before is the goal to have a single file shared by all your projects, and while I can certainly see a few use cases I'd like to know if others face the same situation before merging this in. I'll leave this PR open and if it turns out there's multiple people who'd benefit from this I'm happy to reconsider merging this in.