autotools: Handle case when bindir var in xkbcomp.pc is absent

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107521


Fixes: 72683945 (autotools: Derive xkb configuration from xkbcomp.pc)
Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: vadym.shovkoplias <vadym.shovkoplias@globallogic.com>