configure.ac: ensure RM is set

GNU make predefines RM to rm -f but this is not required by POSIX
so ensure that RM is set.  This fixes "make clean" on OpenBSD.

v2: use AC_CHECK_PROG

Signed-off-by: Jonathan Gray <jsg@jsg.id.au>
CC: "10.6 11.0" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
1 file changed