CMake: Don't use the LZMA imported targets

LibLZMA::LibLZMA was introduced in CMake 3.14.
We need to use the old style link targets. This should also
fix a build issue on macOS when LZMA is not installed into
standard locations.

amends commit 2bf3a5c9

src/private/CMakeLists.txt

 add_subdirectory(protocolgen)
 
+include_directories(${LIBLZMA_INCLUDE_DIRS})
+
 if(NOT XMLLINT_EXECUTABLE)
   message(STATUS "xmllint not found, skipping protocol.xml validation")
 else()
@@ -34,7 +36,7 @@ PUBLIC
     Qt5::Core
     Qt5::DBus
 PRIVATE
-    LibLZMA::LibLZMA
+    ${LIBLZMA_LIBRARIES}
 )
 if (WIN32)
     set(akonadiprivate_LIBS