meson: declare libmpv as a dependency and override it

This allows libmpv users to build it as a subproject easily, i.e. meson
setup build --force-fallback-for=mpv -Dmpv:libmpv=true, if the mpv
source is in the subprojects directory. Mainly useful for development.

meson.build

@@ -1740,6 +1740,10 @@ if get_option('libmpv')
     headers = ['libmpv/client.h', 'libmpv/render.h',
                'libmpv/render_gl.h', 'libmpv/stream_cb.h']
     install_headers(headers, subdir: 'mpv')
+
+    # Allow projects to build with libmpv by cloning into ./subprojects/mpv
+    libmpv_dep = declare_dependency(link_with: libmpv)
+    meson.override_dependency('mpv', libmpv_dep)
 endif
 
 if get_option('cplayer')