* Makefile.am [HAVE_W32_SYSTEM]: Build from w32 directory.
* configure.ac: Don't check for pthread on windows.  Add winsock
library on windows.  Add w32 subdirectory to config files.
* w32/npth.h, w32/npth.c, w32/Makefile.am, w32/npth.def,
w32/npth-config.in, w32/npth.m4: New files.
* tests/Makefile.am: Allow building on w32.