configure.ac: do not set HAVE_DRI(23) when libdrm is missing

configure.ac: do not set HAVE_DRI(23) when libdrm is missing

These conditionals are used to guard both dri modules and loader(s).

Currently if we try to build the gallium swrast dri module (without glx)
on a system that's missing libdrm the build will fail.

v2: Make sure we assign prior to checking the have_libdrm variable.

Cc: 10.6 <mesa-stable@lists.freedesktop.org>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>