[RFC] version: Add compilation info.

Sample output of `nvim --version`:

>NVIM 0.0.0-alpha+201411051757 (compiled Nov  5 2014 20:51:32)
Commit: ebc703f79070a31aaf8b88268e00f7ccb7ae255e
Build type: RelWithDebInfo
Compilation: /usr/bin/cc -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=1 -O2 -g -DNDEBUG -Wall -Wextra -pedantic -Wno-unused-parameter -Wstrict-prototypes -std=gnu99 -DINCLUDE_GENERATED_DECLARATIONS -DHAVE_CONFIG_H -I/home/florian/Projects/neovim/build/config -I/home/florian/Projects/neovim/src -I/home/florian/Projects/neovim/.deps/usr/include -I/home/florian/Projects/neovim/.deps/usr/include -I/home/florian/Projects/neovim/.deps/usr/include/luajit-2.0 -I/home/florian/Projects/neovim/build/src/nvim/auto -I/home/florian/Projects/neovim/build/include

This only tries to reconstruct what CMake does internally, so unfortunately there is no guarantee that the printed command is what CMake actually uses to compile the sources. Still, it could make debugging some issues easier (e.g. #1368).

Currently, includes are not fully correct (always `-I`, but some should be `-isystem`), but otherwise it looks pretty good when comparing it with `build/compile_commands.json`. If someone has an idea for that, please come forward :-)

This uses the "semi-deprecated" `DEFINITIONS` property (CMake doesn't emit a warning, but the [docs](http://www.cmake.org/cmake/help/v3.0/prop_dir/DEFINITIONS.html) mention that `COMPILE_DEFINITIONS` should be used instead), but I didn't find another way to get the `-Wall`/.. flags set with `add_definition()`; `COMPILE_DEFINITIONS` only gives the `-D...` flags.´

Thanks to @zaufi for the idea and the link in http://stackoverflow.com/questions/26699190/get-full-compilation-command-in-cmake/26707132#26707132.
Email chain for reference: https://www.mail-archive.com/cmake@cmake.org/msg42005.html