[CMake] Detecting python modules should be cached

[beanz]

Summary

This requres exec-ing python, which in a trace I ran of the CMake re-configure time took ~2% of the reconfigure time.

[lebedev.ri]

While there it really asks to be rewritten into a proper FindPythonModule.cmake module.

[compnerd]

@lebedev.ri - yeah, that would certainly be nicer to extract this into a separate module, I don't think that it should block this. It would be a great follow up.

llvm/trunk/cmake/config-ix.cmake

 endif()
 
 string(REPLACE " " ";" LLVM_BINDINGS_LIST "${LLVM_BINDINGS}")
 
 function(find_python_module module)
   string(REPLACE "." "_" module_name ${module})
   string(TOUPPER ${module_name} module_upper)
   set(FOUND_VAR PY_${module_upper}_FOUND)
+  if (DEFINED ${FOUND_VAR})
+    return()
+  endif()
 
   execute_process(COMMAND "${PYTHON_EXECUTABLE}" "-c" "import ${module}"
     RESULT_VARIABLE status
     ERROR_QUIET)
 
   if(status)
-    set(${FOUND_VAR} 0 PARENT_SCOPE)
+    set(${FOUND_VAR} OFF CACHE BOOL "Failed to find python module '${module}'")
     message(STATUS "Could NOT find Python module ${module}")
   else()
-    set(${FOUND_VAR} 1 PARENT_SCOPE)
+  set(${FOUND_VAR} ON CACHE BOOL "Found python module '${module}'")
     message(STATUS "Found Python module ${module}")
   endif()
 endfunction()
 
 set (PYTHON_MODULES
   pygments
   # Some systems still don't have pygments.lexers.c_cpp which was introduced in
   # version 2.0 in 2014...