cmake improvements
fix/workaround for gcc internal compiler error
See also http://www.bulletphysics.org/Bullet/phpBB3/viewtopic.php?f=9&t=9636&hilit=compiler+error+gcc

and https://github.com/bulletphysics/bullet3/issues/174
https://github.com/bulletphysics/bullet3/issues/39
Merge branch 'master' of https://github.com/bulletphysics/bullet3