There are a number of cmake variables and cache variables that need to
be passed to all dependencies. This is not only cumbersome, but also
fragile as it's easy to miss adding or removing a flag from a dependency
by accident. Introducing a global variable that controls all builds
makes it much easier to handle our dependencies.

Also fixes the currently broken release workflow as we need to pass the
CMAKE_OSX_ARCHITECTURES variable to all dependencies built with cmake.

This reduces platform-specific differences and the amount of code.

The dependencies aren't set up properly meaning that this will cause a
failure on some systems such as void linux.

Closes https://github.com/neovim/neovim/issues/21982.

Also remove UnibiliumCMakeLists.txt as it's not necessary anymore.

It's simpler to rely on the same build file, especially if there aren't
any platform-specific workarounds like in this case.

It's simpler to let cmake figure out what flags to add to each platforms
to create position independent code rather than handling it ourselves.

Also remove code that sets POSITION_INDEPENDENT_CODE property on SHARED
and MODULE libraries, as it's already on by default.

* build: various cmake refactors and simplifications

- Add STATUS keyword to message to ensure messages are shown in the
  correct order.
- Remove DEPS_CXX_COMPILER as we don't rely on C++ for any of our
  dependencies.
- Simplify how msgpack and luv configure options are constructed.
- Rely on the default installation for luv instead of manually passing
  configure, build and install commands.
- Simplify return code conditional.

* build: remove CMAKE_OSX_ARCHITECTURES_ALT_SEP workaround

CMAKE_OSX_ARCHITECTURES_ALT_SEP was defined as a workaround to prevent
the shell from interpreting `;`, which CMake uses as a list separator.
However, the same thing can be achieved by instead passing
CMAKE_OSX_ARCHITECTURES as a cache variable instead, which is a more
idiomatic way of achieving the same thing.

* build: define CMAKE_BUILD_TYPE before adding it to BUILD_TYPE_STRING

The problem with the current setup is that CMAKE_BUILD_TYPE is defined
after BUILD_TYPE_STRING. BUILD_TYPE_STRING will then be empty on the
first run, meaning that dependencies are built without a build type.
However, since CMAKE_BUILD_TYPE is a cache variable its value will
persist in subsequent runs. On the second run BUILD_TYPE_STRING will
have the correct value, but it's a different value from the ones the
dependencies were built with. This will force some dependencies to be
built again.

Fixes https://github.com/neovim/neovim/issues/21672.

The default cmake installation process for dependencies that use cmake
on all platforms is in general more robust and less verbose, so we rely
on that if possible.