Fix linphone sdk generation for iOS in parallel mode, and make 'sdk' to be the...

Fix linphone sdk generation for iOS in parallel mode, and make 'sdk' to be the default target to avoid confusion. See https://cmake.org/cmake/help/v3.12/command/execute_process.html for more details, especially: "If a sequential execution of multiple commands is required, use multiple execute_process() calls with a single COMMAND argument."