Add check for new namespaced sexp::sexp target

Author: Grumbel

mk/cmake/SuperTux/ProvideSexpcpp.cmake

@@ -7,6 +7,9 @@ endif()
 if(TARGET sexp)
   message(STATUS "Found preinstalled sexp-cpp")
   add_library(LibSexp ALIAS sexp)
+elseif(TARGET sexp::sexp)
+  message(STATUS "Found preinstalled sexp-cpp")
+  add_library(LibSexp ALIAS sexp::sexp)
 else()
   if(USE_SYSTEM_SEXPCPP)
     message(STATUS "Could NOT find sexp-cpp, using external/sexp-cpp fallback")