cmake: do not cache libusb library if libusb-1.0 headers not found

Library found might be an older libusb version.

Closes #158
Allow disabling PROTOBUF support

One dependency less for users that only use DISPMANX grabbing
Fix linking with static Qt Embedded builds

Make sure pthreads and dl get linked when building Hyperion
using a static Qt Embedded library instead of Qt X11
Merge pull request #159 from maxnet/master

Misc build improvements