cmake: Fix conditional linking of fmt

If a simple "if" is used instead, the generated cmake target files will
include the fmt dependency no matter what the surrounding "if" is
evaluated to.
With the generator expression, cmake can detect on generation of the
files if the dependency should be included or not.

src/CMakeLists.txt

@@ -17,10 +17,8 @@ target_link_libraries(
   ${LIBRARY_TARGET}
   PUBLIC Boost::filesystem
   PUBLIC Python::Python
-  PRIVATE ${LUA_LIBRARIES})
-if(${PPPLUGIN_ENABLE_CPP17_COMPATIBILITY})
-  target_link_libraries(${LIBRARY_TARGET} PUBLIC fmt::fmt)
-endif()
+  PRIVATE ${LUA_LIBRARIES}
+  PUBLIC $<$<BOOL:${PPPLUGIN_ENABLE_CPP17_COMPATIBILITY}>:fmt::fmt>)
 target_include_directories(
   ${LIBRARY_TARGET}
   PRIVATE ${LUA_INCLUDE_DIR}