Allow setting existing system LuaJIT prefix

cmake/Modules/GetLuaJIT.cmake

@@ -1,21 +1,33 @@
 include(FindPackageHandleStandardArgs)
 
+if(DEFINED LUAJIT_INSTALL_PREFIX)
+  set(USE_SYSTEM_LUAJIT 1)
+else()
+  set(USE_SYSTEM_LUAJIT 0)
+endif()
+
 set(LUAJIT_VERSION_MAJOR 2)
 set(LUAJIT_VERSION_MINOR 0)
 set(LUAJIT_VERSION_BASE ${LUAJIT_VERSION_MAJOR}.${LUAJIT_VERSION_MINOR})
 set(LUAJIT_VERSION_EXTRA .5)
 set(LUAJIT_BASENAME "LuaJIT-${LUAJIT_VERSION_BASE}${LUAJIT_VERSION_EXTRA}")
+if(NOT USE_SYSTEM_LUAJIT)
 set(LUAJIT_URL "https://luajit.org/download/${LUAJIT_BASENAME}.tar.gz")
 set(LUAJIT_TAR "${PROJECT_BINARY_DIR}/${LUAJIT_BASENAME}.tar.gz")
 set(LUAJIT_SOURCE_DIR "${PROJECT_BINARY_DIR}/${LUAJIT_BASENAME}")
+endif()
 set(LUAJIT_HEADER_BASENAMES lua.h lualib.h lauxlib.h luaconf.h)
 if(WIN32)
+if(NOT USE_SYSTEM_LUAJIT)
   set(LUAJIT_INSTALL_PREFIX "${LUAJIT_SOURCE_DIR}/src")
+endif()
   set(LUAJIT_INCLUDE_DIR "${LUAJIT_INSTALL_PREFIX}")
   set(LUAJIT_LIBRARY_NAME_WE "${LUAJIT_INSTALL_PREFIX}/lua51")
   set(LUAJIT_EXECUTABLE "${LUAJIT_INSTALL_PREFIX}/luajit.exe")
 else()
+if(NOT USE_SYSTEM_LUAJIT)
   set(LUAJIT_INSTALL_PREFIX "${PROJECT_BINARY_DIR}/luajit")
+endif()
   set(LUAJIT_INCLUDE_DIR "${LUAJIT_INSTALL_PREFIX}/include/luajit-${LUAJIT_VERSION_BASE}")
   set(LUAJIT_LIBRARY_NAME_WE "${LUAJIT_INSTALL_PREFIX}/lib/libluajit-5.1")
   set(LUAJIT_EXECUTABLE "${LUAJIT_INSTALL_PREFIX}/bin/luajit-${LUAJIT_VERSION_BASE}${LUAJIT_VERSION_EXTRA}")
@@ -33,12 +45,14 @@ string(CONCAT
   "${CMAKE_SHARED_LIBRARY_SUFFIX}"
 )
 
+if(NOT USE_SYSTEM_LUAJIT)
 file(DOWNLOAD "${LUAJIT_URL}" "${LUAJIT_TAR}")
 
 execute_process(
   COMMAND "${CMAKE_COMMAND}" -E tar xzf "${LUAJIT_TAR}"
   WORKING_DIRECTORY ${PROJECT_BINARY_DIR}
 )
+endif()
 
 foreach(LUAJIT_HEADER ${LUAJIT_HEADER_BASENAMES})
   list(APPEND LUAJIT_INSTALL_HEADERS "${LUAJIT_INCLUDE_DIR}/${LUAJIT_HEADER}")
@@ -54,6 +68,7 @@ if(UNIX AND NOT APPLE)
   )
 endif()
 
+if(NOT USE_SYSTEM_LUAJIT)
 if(WIN32)
   add_custom_command(
     OUTPUT ${LUAJIT_STATIC_LIBRARY} ${LUAJIT_SHARED_LIBRARY_PATHS} ${LUAJIT_EXECUTABLE} ${LUAJIT_INSTALL_HEADERS}
@@ -109,6 +124,7 @@ else()
     VERBATIM
   )
 endif()
+endif()
 
 foreach(LUAJIT_HEADER ${LUAJIT_HEADER_BASENAMES})
   list(APPEND LUAJIT_HEADERS ${PROJECT_BINARY_DIR}/include/terra/${LUAJIT_HEADER})
@@ -134,7 +150,7 @@ foreach(LUAJIT_HEADER ${LUAJIT_HEADER_BASENAMES})
   )
 endforeach()
 
-if(TERRA_SLIB_INCLUDE_LUAJIT)
+if(TERRA_SLIB_INCLUDE_LUAJIT AND NOT USE_SYSTEM_LUAJIT)
   set(LUAJIT_OBJECT_DIR "${PROJECT_BINARY_DIR}/lua_objects")
   file(MAKE_DIRECTORY "${LUAJIT_OBJECT_DIR}")