qwt: add optional opengl support

qwt should only build the opengl support code if Qt is configured with
opengl support, so add a suboption for it similar to svg/mathml with the
needed dependencies.

Fixes http://autobuild.buildroot.net/results/597/5977bd77342e28ae9783f9f9edfcf84eefd081b6/



Signed-off-by: Peter Korsgaard <peter@korsgaard.com>