meson.add_dist_script() allowd in subprojects

meson.add_dist_script() can now be invoked from a subproject, it was a hard error in earlier versions. Subproject dist scripts will only be executed when running meson dist --include-subprojects. MESON_PROJECT_SOURCE_ROOT, MESON_PROJECT_BUILD_ROOT and MESON_PROJECT_DIST_ROOT environment variables are set when dist scripts are run. They are identical to MESON_SOURCE_ROOT, MESON_BUILD_ROOT and MESON_DIST_ROOT for main project scripts, but for subproject scripts they have the path to the root of the subproject appended, usually subprojects/<subproject-name>.

Note that existing dist scripts likely need to be modified to use those new environment variables instead of MESON_DIST_ROOT to work properly when used from a subproject.