Currently you can't use GUI_EXE without BUILD_GUI, but BUILD_GUI also
requires the yarn command (even though it will never use it when GUI_EXE
is set).
This commit fixes it:
- Make `GUI_EXE` a windows-only top-level project options, rather than
being guarded by `BUILD_GUI`.
- Make `BUILD_GUI` control *building* the GUI instead of bundling it.
- GUI_EXE and BUILD_GUI are now mutually exclusive.
The `sign` target on macos was not working properly -- the signing
script would run before the build is finished. This was caused by
cmake/macos.cmake having an `if(BUILD_GUI)`, but BUILD_GUI isn't defined
as an option until cmake/gui.cmake, which hadn't been included yet where
macos.cmake was included.
This extracts just the `option(BUIL_GUI)` from gui.cmake into a separate
gui-option.cmake file that we can load earlier to fix it.
While here I also noticed the GUI_EXE setting was defined as an option,
but isn't actually a boolean value, as an option, but isn't actually a
boolean value, so fixed it by making it a `set(... CACHE FILEPATH ...)`.
