build.sh: use accurate values for HOST_OS from config.guess
build.sh: use accurate values for HOST_OS from config.guess

Using *Linux* rather than Linux is misleading to the user as it
implies there are a large number of possibilities.

In addition, FreeBSD | NetBSD | OpenBSD is more informative than *BSD*
which is not used in config.guess.

Use the values from config.guess which are known to be accurate.

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