gettid impl in macOS and cygwin64

[suzp1984]

srs/trunk/src/app/srs_app_threads.cpp

Lines 39 to 48 in 954b1b7

	#if defined(SRS_OSX) || defined(SRS_CYGWIN64)
	pid_t gettid() {
	return 0;
	}
	#else
	#if __GLIBC__ == 2 && __GLIBC_MINOR__ < 30
	#include <sys/syscall.h>
	#define gettid() syscall(SYS_gettid)
	#endif
	#endif

I already met same issue in another project:
https://github.com/signalwire/libks/pull/205/files

So why not add this patch here?

[winlinvip]

Please do not open an issue, unless you really need to open a bug report.

1. Open a discussion if want to discuss.
2. Open a pull request if want to improve, where we can discuss at.

I will delete this issue.