Add support for YouCompleteMe vim plugin.

Needs the following tools to be useful:
YouCompleteMe: https://github.com/Valloric/YouCompleteMe
Bear: https://github.com/rizsotto/Bear

* .ycm_extra_conf.py: Add config file for YouCompleteMe plugin, which
    looks for compile flags in compile_commands.json in the srcdir.

* Makefile.in: Add new compile-commands target that produces
    compile_commands.json by using bear.

* trunk/ (svn:ignore): Ignore compile_commands.json file.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1476374 13f79535-47bb-0310-9956-ffa450edef68