Add LICENSE. fix #878. PR #883

- change to Apache 2.0
- include Vim license in LICENSE
- upate README
Fix #838: build with "USE_BUNDLED=OFF" fails to find dependencies

Stop forcing some platform setting that are really intended to be used
for Travis CI.  Under other systems, like Arch Linux, it prevents
dependencies from being correctly located.
Merge pull request #844:

'Fix #838: build with "USE_BUNDLED=OFF" fails to find dependencies'
Fix #867: cmake doesn't issue `-isystem` for versioned gcc

CMake purposefully disables the use of the `-isystem` flag on Apple
platforms, but it's overly blunt with the detection.  Apple's compilers
have supported the flag since at least 10.4.  Let's force the switch to
be on when gcc/g++ is detected on an Apple platform to reduce the
warnings out of the msgpack-related bits.
Merge #887 'Fix #867: cmake doesn't issue `-isystem` for versioned gcc'