[BUG] Build with -DENABLE_STDCXX_SYNC=ON on Linux fails

[maxsharabayko]

Building the latest SRT version with -DENABLE_STDCXX_SYNC=ON fails on Linux (e.g. Ubuntu 20.04 LTS).

It is still required to link pthreads to the target application.

/usr/bin/ld: CMakeFiles/srt-tunnel.dir/apps/srt-tunnel.cpp.o: undefined reference to symbol 'pthread_create@@GLIBC_2.2.5'
//lib/x86_64-linux-gnu/libpthread.so.0: error adding symbols: DSO missing from command line
collect2: error: ld returned 1 exit status
CMakeFiles/srt-tunnel.dir/build.make:107: recipe for target 'srt-tunnel' failed
make[2]: *** [srt-tunnel] Error 1
CMakeFiles/Makefile2:68: recipe for target 'CMakeFiles/srt-tunnel.dir/all' failed
make[1]: *** [CMakeFiles/srt-tunnel.dir/all] Error 2
Makefile:129: recipe for target 'all' failed
make: *** [all] Error 2