Fix "unknown CMake command "check_library_exists" (#4517)

Adds CMakeLists.txt include to fix this issue:

$ cmake . -DBUILD_SERVER=TRUE -DBUILD_CLIENT=FALSE \
   -DCMAKE_BUILD_TYPE=Release -DENABLE_CURL=TRUE -DENABLE_CURSES=FALSE \
   -DENABLE_FREETYPE=TRUE -DENABLE_GETTEXT=FALSE \
   -DENABLE_POSTGRESQL=FALSE -DENABLE_SOUND=FALSE -DENABLE_LUAJIT=TRUE \
   -DIRRLICHT_SOURCE_DIR=$HOME/irrlicht-1.8.4 -LH
...
CMake Error at src/CMakeLists.txt:339 (check_library_exists):
  Unknown CMake command "check_library_exists".
...