Fix CMake compile definitions

Source/Native/CMakeLists.txt

@@ -17,7 +17,7 @@ if (ENET_STATIC)
     if (NOT ENET_LZ4)
         add_library(enet_static STATIC library.c)
     else()
-        add_definitions(ENET_LZ4)
+        add_definitions(-DENET_LZ4)
         add_library(enet_static STATIC library.c lz4/lz4.c)
     endif()
 
@@ -30,7 +30,7 @@ if (ENET_SHARED)
     if (NOT ENET_LZ4)
         add_library(enet SHARED library.c)
     else()
-        add_definitions(ENET_LZ4)
+        add_definitions(-DENET_LZ4)
         add_library(enet SHARED library.c lz4/lz4.c)
     endif()