The variables are not marked as advanced, thus they appear in e.g. `ccmake`.

This is needed specifically for the second signature of
add_custom_command, which appends an operation to an existing target.
This will prevent the cmake warning CMP0175.

Reference: https://cmake.org/cmake/help/latest/policy/CMP0175.html

Problem: Currently Neovim only searches for lpeg.dylib, liblpeg.dylib,
etc. on MacOS, but a normal installation of lpeg will produce lpeg.so
instead.  There are explicit workarounds for this by modifying lpeg's
package in Homebrew, and will shortly be another in spack.

Solution: also search for lpeg.so even if that's not the platform
default, because Lua and luarocks use it anyway.

Forcing the build type to be one of the default types was intended to
prevent usage of custom build types which risks unpredictability.
Unfortunately, many distributions patch this check out so they can use
a custom type anyway, making the check moot in the first place. Instead,
allow custom build types but don't try to accomodate them in the code:
we're making the assumption that anyone who wants to use a custom build
type will be able to set any additional necessary flags needed for their
specific system.

Policy CMP0092 is by default on for minimum cmake versions above 3.15.

Problem: Installing treesitter parser is hard (harder than
climbing to heaven).

Solution: Add optional support for wasm parsers with `wasmtime`.

Notes:

* Needs to be enabled by setting `ENABLE_WASMTIME` for tree-sitter and
  Neovim. Build with
  `make CMAKE_EXTRA_FLAGS=-DENABLE_WASMTIME=ON
  DEPS_CMAKE_FLAGS=-DENABLE_WASMTIME=ON`
* Adds optional Rust (obviously) and C11 dependencies.
* Wasmtime comes with a lot of features that can negatively affect
  Neovim performance due to library and symbol table size. Make sure to
  build with minimal features and full LTO.
* To reduce re-compilation times, install `sccache` and build with
  `RUSTC_WRAPPER=<path/to/sccache> make ...`

Problem: Adding support for modern Nvim features (reflow, OSC 8, full
utf8/emoji support) requires coupling libvterm to Nvim internals
(e.g., utf8proc).

Solution: Vendor libvterm at v0.3.3.

utf8proc contains all the data which is currently in
unicode_tables.generated.h internally, but in quite a different format.
Ideally unicode_tables.generated.h should be removed as well so we rely
solely on utf8proc. We want to avoid a situation where the possibility
of unicode mismatch occurs, e.g a distro using both unicode 12 and
unicode 13.