winpr: fixed build warnings

* moved pthread_tryjoin_np to toplevel CMakeLists.txt
* removed duplicated -DHAVE_PTHREAD_GNU_EXT
Merge pull request #1596 from bmiklautz/pthread_ext

winpr: fixed build warnings
Merge branch 'master' of github.com:FreeRDP/FreeRDP
libfreerdp-core: add external certificate management, pass X509 PEM certificate through client callback
channels/rdpsnd: add option for configuring audio quality mode
channels/rdpsnd: add wlog debug output
libfreerdp-crypto: don't report SSL_ERROR_SYSCALL with errno value 0 as error
Merge remote-tracking branch 'upstream/master'