Allow custom 3rd party libs missing option fix (#360)

* Allow overriding the game engine external libraries with ones provided by the game.

* Fix for missing BUILD_EXT_UNZIP option.

Author: rh101

CMakeLists.txt

@@ -23,6 +23,7 @@ option(BUILD_EXT_MD5 "Build with internal md5 support" ON)
 option(BUILD_EXT_SQLITE "Build with internal SQLite support" ON)
 option(BUILD_EXT_ZLIB "Build with internal zlib support" ON)
 option(BUILD_EXT_CURL "Build with internal curl support" ON)
+option(BUILD_EXT_UNZIP "Build with internal unzip support" ON)
 
 add_library(external empty.cpp)