correct python3 variable

Author: AnastaZIuk

src/nbl/builtin/utils.cmake

@@ -9,8 +9,8 @@ macro(LIST_BUILTIN_RESOURCE _BUNDLE_NAME_ _LBR_PATH_)
 endmacro()
 
 function(ADD_CUSTOM_BUILTIN_RESOURCES _TARGET_NAME_ _BUNDLE_NAME_ _BUNDLE_SEARCH_DIRECTORY_ _NAMESPACE_ _OUTPUT_DIRECTORY_)
-	if(NOT DEFINED Python3_EXECUTABLE)
-		message(FATAL_ERROR "Python3_EXECUTABLE must be defined!")
+	if(NOT DEFINED _Python3_EXECUTABLE)
+		message(FATAL_ERROR "_Python3_EXECUTABLE must be defined - call find_package(Python3 COMPONENTS Interpreter REQUIRED)")
 	endif()
 
 	file(MAKE_DIRECTORY "${_OUTPUT_DIRECTORY_}")
@@ -62,8 +62,8 @@ function(ADD_CUSTOM_BUILTIN_RESOURCES _TARGET_NAME_ _BUNDLE_NAME_ _BUNDLE_SEARCH
 
 	add_custom_command(
 		OUTPUT "${NBL_BUILTIN_RESOURCES_HEADER}" "${NBL_BUILTIN_RESOURCE_DATA_SOURCE}"
-		COMMAND "${Python3_EXECUTABLE}" "${NBL_BUILTIN_HEADER_GEN_PY}" "${NBL_BUILTIN_RESOURCES_HEADER}" "${_BUNDLE_SEARCH_DIRECTORY_}" "${NBL_RESOURCES_LIST_FILE}" "${_NAMESPACE_}" "${_GUARD_SUFFIX_}"
-		COMMAND "${Python3_EXECUTABLE}" "${NBL_BUILTIN_DATA_GEN_PY}" "${NBL_BUILTIN_RESOURCE_DATA_SOURCE}" "${_BUNDLE_SEARCH_DIRECTORY_}" "${NBL_RESOURCES_LIST_FILE}" "${_NAMESPACE_}" "${NBL_BS_HEADER_FILENAME}"
+		COMMAND "${_Python3_EXECUTABLE}" "${NBL_BUILTIN_HEADER_GEN_PY}" "${NBL_BUILTIN_RESOURCES_HEADER}" "${_BUNDLE_SEARCH_DIRECTORY_}" "${NBL_RESOURCES_LIST_FILE}" "${_NAMESPACE_}" "${_GUARD_SUFFIX_}"
+		COMMAND "${_Python3_EXECUTABLE}" "${NBL_BUILTIN_DATA_GEN_PY}" "${NBL_BUILTIN_RESOURCE_DATA_SOURCE}" "${_BUNDLE_SEARCH_DIRECTORY_}" "${NBL_RESOURCES_LIST_FILE}" "${_NAMESPACE_}" "${NBL_BS_HEADER_FILENAME}"
 		COMMENT "Generating built-in resources"
 		DEPENDS ${NBL_DEPENDENCY_FILES}
 		VERBATIM