Prepare openvsp for CMake 3.10

[adridg]

Summary

Uses libxml, without looking for it; currently this builds,
but newer cmake doesn't set up the variables the same way
(I guess some accidental import is happening in cmake 3.9),
so link fails with libxml-not-found. To fix, just look for
libxml.

Test Plan

See http://package23.nyi.freebsd.org/data/103i386-default-PR223872/2017-11-27_14h43m28s/logs/errors/openvsp-3.13.3_1.log for error log; with this patch the same failure no longer happens; harmless in CMake 3.9.

[adridg]

Will do cmake update plus all these related fixes in one go in PR 223872