The FindLua CMake script would previously set LUA_FOUND if the directory
containing the source code of Lua existed without checking if it was
actually part of the build. This could lead to compilation errors if
CAMPVIS_BUILD_LIB_LUA was disabled.

Add an additional check to FindLua to only set LUA_FOUND if
CAMPVIS_BUILD_LIB_LUA is ON and the directory containing the source code
of Lua exists.

References #1