gettid impl in macOS and cygwin64

[Jacob Su]

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?

[Winlin]

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.