build-sys: fix "remove the g in git describe"

build-sys: fix "remove the g in git describe"

The "g" was not always removed and "-rc*" follow-ups were handled
incorrectly.

So now after last commits we have these versions:

 git describe            -> blkid -V
 --------------------------------------------------------------
 v2.29-rc4               -> 2.29-rc4          (libblkid 2.29.0)
 v2.29-rc4-3-g4c8928d    -> 2.29-rc4-3-4c89   (libblkid 2.29.0)

 v2.29                   -> 2.29              (libblkid 2.29.0)
 v2.29-78-g4c8928d       -> 2.29.78-4c89      (libblkid 2.29.78)

 v2.29.1                 -> 2.29.1            (libblkid 2.29.1)
 v2.29.1-4-g4c8928d      -> 2.29.1.4-4c89     (libblkid 2.29.1)

 v2.29.1-rc2             -> 2.29.1-rc2        (libblkid 2.29.1)
 v2.29.1-rc2-3-g4c8928d  -> 2.29.1-rc2-3-4c89 (libblkid 2.29.1)

Signed-off-by: Ruediger Meier <ruediger.meier@ga-group.nl>