buildconf: exec autoreconf to avoid additional process (libssh2#512)

Also make buildconf exit with the return code of autoreconf.

Follow up to libssh2#224

Author: mback2k

buildconf

@@ -5,4 +5,4 @@ echo "*** Do not use buildconf. Instead, just use: autoreconf -fi" >&2
 echo "*** Doing it for you now, but buildconf may disapear in the future." >&2
 echo "***" >&2
 
-${AUTORECONF:-autoreconf} -fi "${@}"
+exec ${AUTORECONF:-autoreconf} -fi "${@}"