protobuf: reuse already protoc'd files from linuxcnc build directory

(x86 tested so far only)
protobuf: package pre-protoc'd C++ files and headers

NB: this will not work once proto files are changed - ok for now

eventually just point PROTOGEN in QtQuickVCP.pro to the
linuxcnc/src/middleware/generated directory to stay in sync
Merge remote-tracking branch 'mah/master'

Conflicts:
	QtQuickVcp.pro
	protobuf.pri