Initially done in 28e59cb22, but does not appear to be necessary
anymore.

Uses UV_EEXIST directly, just like UV_ENOENT.

Now that uv.h is directly being included, pre-processing of
test/includes/pre/uv.h fails on Linux with

    In file included from «SRCDIR»/neovim/test/includes/pre/uv.h:1:
    In file included from /usr/include/uv.h:62:
    /usr/include/uv/unix.h:72:11: fatal error: 'uv/pthread-barrier.h' file not found
    # include "uv/pthread-barrier.h"
              ^~~~~~~~~~~~~~~~~~~~~~
    1 error generated.

This happens because we're missing -D_GNU_SOURCE (part of ${gen_cdefs}),
which makes the pthread_barrier_* functionality visible.

libuv users are only supposed to directly include uv.h.  In v1.21.0, all
the uv-*.h headers were renamed to uv/*.h, which caused the unit tests
to fail with

    [123/125] Generating post/uv-errno.h
    FAILED: test/includes/post/uv-errno.h
    cd «SRCDIR»/src/neovim/build/test/includes && /usr/bin/clang -std=c99 -E -P «SRCDIR»/src/neovim/test/includes/pre/uv-errno.h -I/usr/include -I/usr/include -o «SRCDIR»/neovim/build/test/includes/post/uv-errno.h
    «SRCDIR»/src/neovim/test/includes/pre/uv-errno.h:1:10: error: 'uv-errno.h' file not found with <angled> include; use "quotes" instead
    #include <uv-errno.h>
             ^~~~~~~~~~~~
             "uv-errno.h"

The intention of the file is to extend libuv's error constants with more
values used by the unit tests.  This can just as easily be achieved
without poking into pseudo-private header files.

Enabling CMake's USE_FOLDERS option and adding the FOLDER property to
targets allows some IDEs to list the targets in an organized
hierarchy of folders.

We need to add the SDK includes to the preprocessing step, otherwise it
will fail to resolve the system includes such as sys/stat.h and fcntl.h.

POSIX.1-2008[1] says that the latter should be used, and all of our
supported platforms would seem to support this scheme, apparently even
Windows[2].

[1]: http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/fcntl.h.html
[2]: https://msdn.microsoft.com/en-us/library/z0kc8e3z.aspx

Replaced old unit tests for errno with libuv error codes UV_ENOENT
and UV_EEXIST (for os_open and os_getperms).

Added libuv include path to test/includes compiler calls - needed
to get hold of libuv headers.

This achieves several goals:

 * Less reliance on scripts so we have better portability to Windows
   (though we still have a ways to go for proper Windows support).
   Luajit, luarocks, moonscript, and busted are all installed via CMake
   now.
 * Trying to make use of pkg-config to get the correct libraries.  The
   latest libuv is still broken in this regard, but we'll at least be in
   a position to use it.
 * Allow the use of Ninja or make.  The former runs faster in many
   environments, and automatically makes use of parallel builds.

This also allows for system installed dependencies--though not through
the Makefile just yet--and adds support for FreeBSD.

This also make us build libuv and luajit as static libraries only, since
we're only concerned about having static libraries for our bundled
dependencies.