build.sh: verify PREFIX is a directory and is writable
build.sh: verify PREFIX is a directory and is writable

This will catch the case where user forgets to set PREFIX
and does not have write permission in the /usr/local default location

Reviewed-by: Trevor Woerner <8835777cd84fba3d7873eabbbe1f2de2d3f25b00@gmail.com>
Reviewed-by: Matt Dew <1fa2ef4755a9226cb9a0a4840bd89b158ac71391@osource.org>
Acked-by: Daniel Stone <3d0f3b9ddcacec30c4008c5e030e6c13a478cb4f@fooishbar.org>
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