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>

configure.ac

@@ -107,6 +107,8 @@ AC_SYS_LARGEFILE
 LT_PREREQ([2.2])
 LT_INIT([disable-static])
 
+AC_CHECK_PROG(RM, rm, [rm -f])
+
 AX_PROG_BISON([],
               AS_IF([test ! -f "$srcdir/src/glsl/glcpp/glcpp-parse.c"],
                     [AC_MSG_ERROR([bison not found - unable to compile glcpp-parse.y])]))