Dummy dependencies are not required to execute a subproject
automatically for providing a program, nor do you need to explicitly
call subproject() to do that.
A `[provide]` section in the wrap file is enough.
Instead of requiring sassc to be installed add meson subprojects
which build libsass and sassc (currently both forks of mine, tested
under linux/mingw/msvc) when needed.
This allows us to drop the generated .css files and build scripts from git.
See #1502
