build.sh: use more descriptive variable names for check_writable_dir
build.sh: use more descriptive variable names for check_writable_dir

Reported-by: Peter Hutterer <b9c50af7afa7642f6f854434cda9ffe54ece8b85@who-t.net>

Reviewed-by:  Peter Hutterer <b9c50af7afa7642f6f854434cda9ffe54ece8b85@who-t.net>
Signed-off-by: Gaetan Nadon <8a8cc541338200f67d4d0bcb65c78b6166649462@videotron.ca>

freedesktop-unofficial-mirror/xorg__util__modular,freedesktop-unofficial-mirror/xorg__util__modular,freedesktop-unofficial-mirror/xorg__util__modular