Support external builds of python bindings, gui, ctbgui and moench stuff (#440)

Use already installed version of the slsDetectorPackage. Assumes that the library has already been built and installed either on a system wide location or pointed to by CMAKE_PREFIX_PATH

slsDetectorSoftware/CMakeLists.txt

@@ -17,6 +17,8 @@ add_library(slsDetectorObject OBJECT
 target_include_directories(slsDetectorObject PUBLIC
     "$<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>"
     "$<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>"
+    PRIVATE
+    ${SLS_INTERNAL_RAPIDJSON_DIR}
 )
 
 target_link_libraries(slsDetectorObject