buildconf: exec autoreconf to avoid additional process

Also make buildconf exit with the return code of autoreconf.

Reviewed-by: Daniel Stenberg

Follow up to #5853
Closes #5890

buildconf

@@ -1,4 +1,4 @@
 #!/bin/sh
 
 echo "*** Do not use buildconf. Instead, just use: autoreconf -fi" >&2
-${AUTORECONF:-autoreconf} -fi "${@}"
+exec ${AUTORECONF:-autoreconf} -fi "${@}"