winpr: fixed build warnings

* moved pthread_tryjoin_np to toplevel CMakeLists.txt
* removed duplicated -DHAVE_PTHREAD_GNU_EXT
winpr: disabled schannel test

not practicable for now since it requires local key setup
Merge pull request #1596 from bmiklautz/pthread_ext

winpr: fixed build warnings
client/common: fixed TestClientRdpFile test

Unified ascii and unicode string handling.
Merge pull request #1597 from bmiklautz/tests

Fixed failing tests
build: preload for debian squeeze added