cmake: Allow building without Luajit

By default Neovim searched a Luajit instalation and linked against
the luajit library.

In practice Neovim only requires luajit to run the unit tests. All other
targets only require lua and the correct lua modules. This commit:

1. Remove the strict dependency on Luajit
2. Makes the unittest target depend on the lua 'ffi' module.
   If the module is not available the target is not enabled
   and a message is displayed.

CMakeLists.txt

@@ -300,9 +300,6 @@ include_directories(SYSTEM ${LIBUV_INCLUDE_DIRS})
 find_package(Msgpack 1.0.0 REQUIRED)
 include_directories(SYSTEM ${MSGPACK_INCLUDE_DIRS})
 
-find_package(LuaJit REQUIRED)
-include_directories(SYSTEM ${LUAJIT_INCLUDE_DIRS})
-
 find_package(Unibilium REQUIRED)
 include_directories(SYSTEM ${UNIBILIUM_INCLUDE_DIRS})
 
@@ -477,6 +474,8 @@ if(BUSTED_PRG)
   add_custom_target(benchmark-prereqs
     DEPENDS ${BENCHMARK_PREREQS})
 
+  check_lua_module(${LUA_PRG} "ffi" LUA_HAS_FFI)
+  if(LUA_HAS_FFI)
     add_custom_target(unittest
       COMMAND ${CMAKE_COMMAND}
         -DBUSTED_PRG=${BUSTED_PRG}
@@ -489,6 +488,10 @@ if(BUSTED_PRG)
         -P ${PROJECT_SOURCE_DIR}/cmake/RunTests.cmake
       DEPENDS ${UNITTEST_PREREQS}
       ${TEST_TARGET_ARGS})
+  else()
+    message(WARNING "The Luajit ffi is not available in ${LUA_PRG}"
+	    ", disabling unit tests")
+  endif()
 
   add_custom_target(functionaltest
     COMMAND ${CMAKE_COMMAND}

cmake/LuaHelpers.cmake

@@ -8,8 +8,6 @@ function(check_lua_module LUA_PRG_PATH MODULE RESULT_VAR)
     RESULT_VARIABLE module_missing
     ERROR_QUIET)
   if(module_missing)
-    message(STATUS
-      "[${LUA_PRG_PATH}] The '${MODULE}' lua package is required for building Neovim")
     set(${RESULT_VAR} False PARENT_SCOPE)
   else()
     set(${RESULT_VAR} True PARENT_SCOPE)
@@ -29,6 +27,8 @@ function(check_lua_deps LUA_PRG_PATH MODULES RESULT_VAR)
   foreach(module ${MODULES})
     check_lua_module(${LUA_PRG_PATH} ${module} has_module)
     if(NOT has_module)
+      message(STATUS
+        "[${LUA_PRG_PATH}] The '${module}' lua package is required for building Neovim")
       set(${RESULT_VAR} False PARENT_SCOPE)
       return()
     endif()

src/nvim/CMakeLists.txt

@@ -231,7 +231,6 @@ endif()
 list(APPEND NVIM_LINK_LIBRARIES
   ${LIBUV_LIBRARIES}
   ${MSGPACK_LIBRARIES}
-  ${LUAJIT_LIBRARIES}
   ${LIBVTERM_LIBRARIES}
   ${LIBTERMKEY_LIBRARIES}
   ${UNIBILIUM_LIBRARIES}