buildroot

autoconf: don't append -dirty to version number if built in buildroot git tree

Autoconf would append -dirty to it's version number, causing build breakage
with packages explicitly requesting autoconf 2.65, if built in a subdir
of a git tree with uncommitted changes. This is a relatively common
situation when developing on BR.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>