[CMake] Build library only if sources are detected.

src/CMakeLists.txt

@@ -48,11 +48,12 @@ IF(LUA5_1_FOUND)
       multibody/parser/lua/lua_tables.cpp
       multibody/parser/lua.cpp
     )
+  SET(BUILD_MAIN_LIBRARY True)
 ENDIF(LUA5_1_FOUND)
 
 SET(${PROJECT_NAME}_SOURCES ${${PROJECT_NAME}_MULTIBODY_PARSER_LUA_SOURCES}) 
 
-IF (UNIX )  
+IF (UNIX AND BUILD_MAIN_LIBRARY)
     # Create target libpinocchio.so
     ADD_LIBRARY ( ${PROJECT_NAME} SHARED ${HEADERS} ${${PROJECT_NAME}_SOURCES} )
 		SET_TARGET_PROPERTIES( ${PROJECT_NAME} PROPERTIES LINKER_LANGUAGE CXX)