In Windows Lua's os.tmpname() returns relative paths starting with \s,
prepend them with $TEMP to generate a valid path.

In OS X os.tmpname() returns paths in '/tmp' but they should be in
'/private/tmp'. We cannot use os_name() for platform detection because
some tests use tempname() before nvim is spawned, instead use one of the
following:

1. Set SYSTEM_NAME environment variable before calling the tests, it
   is set from CMAKE_SYSTEM_NAME(i.e. uname -s or 'Windows')
2. Call uname -s
3. Assume windows

- NVIM_RPLUGIN_MANIFEST
- XDG_CONFIG_HOME
- XDG_DATA_HOME

Using /MT was causing issues when building luarocks, revert it, use the
dynammic runtime and generate release DLLs for the dependencies.
Some refactoring was required because for linking cmake looks for the
import libraries (.lib) but on runtime executables we need the .dll files
to be in the same folder.

The DLLs are placed in the bin/ folder in order for nvim.exe to run
during the build and tests. The install target installs the DLLs with
the nvim binary - uses GetPrerequisites to find runtime DLLs.

Some minor issues that required adjustments:
- [MSVC] FindMsgpack.cmake now looks for msgpack_import.lib instead of
  msgpack.lib
- The lua-client fails to find libuv.lib, instead it looks for uv.lib,
  added second copy of the file to the install command.
- [MSVC] CMAKE_BUILD_TYPE affects the output paths, default to Release.

Part of these changes are credited to @jasonwilliams200OK who fixed the
third-party recipes to consistently use the same build type.

Otherwise, busted may run a different interpreter than the one we
detected, without the capabilities we expect.  (For instance, we might
have detected the luajit interpreter, but busted might run the lua
interpreter, without the ffi module.)

By default Neovim searched a Luajit instalation and linked against
the luajit library.

In practice Neovim only requires luajit to run the unit tests. All other
targets only require lua and the correct lua modules. This commit:

1. Remove the strict dependency on Luajit
2. Makes the unittest target depend on the lua 'ffi' module.
   If the module is not available the target is not enabled
   and a message is displayed.

For that, make luatest ignore the preload.lua files.

For instance to only lint "edit.c"
make lint LINT_FILE="src/nvim/edit.c"

Avoid duplicating information by using the Msgpack_FIND_VERSION variable
exported by find_package() inside FindMsgpack's pkg_search_module call.

By default, find_library() searches all directories for one possible
name and then looks for the next name.  To make sure we're building
against the same headers and libraries, look for all names in a
directory before moving to the next one.