CMake: Only require finding Lua libs via find_package on Windows

Introduced in #346, fixes #488

CMakeLists.txt

@@ -104,11 +104,29 @@ if (LUA)
 else (LUA)
   if (LUA_BUILD_TYPE STREQUAL System)
     if (USE_LUAJIT)
-      find_package(LuaJIT REQUIRED)
+      # We only link the libs on Windows, so find_package fully succeeding
+      # is only required on Windows
+      if (WIN32)
+        find_package(LuaJIT REQUIRED)
+        link_directories(${LUAJIT_LIBRARIES})
+      else()
+        find_package(LuaJIT)
+      endif()
+      if(NOT LUAJIT_INCLUDE_DIR)
+        message( FATAL_ERROR "Failed to find LuaJIT headers. Variable `LUAJIT_INCLUDE_DIR' expected to be defined.")
+      endif()
       include_directories(${LUAJIT_INCLUDE_DIR})
-      link_directories(${LUAJIT_LIBRARIES})
     else (USE_LUAJIT)
-      find_package(Lua REQUIRED)
+      # We only link the libs on Windows, so find_package fully succeeding
+      # is only required on Windows
+      if (WIN32)
+        find_package(Lua REQUIRED)
+      else()
+        find_package(Lua)
+      endif()
+      if(NOT LUA_INCLUDE_DIR)
+        message( FATAL_ERROR "Failed to find Lua headers. Variable `LUA_INCLUDE_DIR' expected to be defined.")
+      endif()
       include_directories(${LUA_INCLUDE_DIR})
     endif (USE_LUAJIT)