report version number in user agent string

refs #628
make sure it also works when there are no git tags
don't print more information when a tag actually is available
support fewer parameters
add missing gyp file
Merge pull request #705 from mapbox/user-agent-os

Include operating system and actual version number in User Agent string
Tidy up HeadlessView
don't include the source file in every library
also install the version header
Merge pull request #709 from mapbox/version-header-only

Include version symbols only in mbgl-core library
fix typo
Merge pull request #708 from mapbox/headless-tidy

Tidy up HeadlessView
wrap mbgl-core and mbgl-linux in -Wl,--start-group/-Wl,--end-group
Merge pull request #707 from mapbox/start-group

wrap mbgl-core and mbgl-linux in -Wl,--start-group/-Wl,--end-group
disable uv_thread_self() asserts, ref #623