Fix CPack packaging paths

Fixes: #112

Author: semlanik

.ci/Dockerfile.ubuntu_deb

@@ -2,5 +2,5 @@ FROM qtprotobuf/ubuntu-latest-builtin-qt:latest
 ADD . /sources
 RUN mkdir -p /build && mkdir -p /artifacts
 WORKDIR /build
-RUN cmake ../sources -DQT_PROTOBUF_MAKE_TESTS=FALSE -DQT_PROTOBUF_MAKE_EXAMPLES=FALSE -DCMAKE_BUILD_TYPE=Release
-RUN cpack -G DEB ..
+RUN cmake ../sources -DQT_PROTOBUF_MAKE_TESTS=FALSE -DQT_PROTOBUF_MAKE_EXAMPLES=FALSE -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=/usr
+RUN cpack -G DEB

cmake/packaging.cmake

@@ -1,6 +1,7 @@
 set(CPACK_PACKAGE_CONTACT "Alexey Edelev <semlanik@gmail.com>")
 set(CPACK_PACKAGE_DESCRIPTION "gRPC and Protobuf generator and bindings for Qt framework")
 set(CPACK_PACKAGE_NAME "qtprotobuf")
+set(CPACK_PACKAGING_INSTALL_PREFIX "${CMAKE_INSTALL_PREFIX}")
 
 include("${QT_PROTOBUF_CMAKE_DIR}/deb.cmake")
 include("${QT_PROTOBUF_CMAKE_DIR}/rpm.cmake")