MinGW build: Fix sleep() substitution

MinGW does not natively support the POSIX sleep() function so we have
had an override that was a part of the GR_PWIN32 macro and included in
the generated config.h file.  When compiling for Windows on POSIX using
MinGW, Autotools will detect sleep() and set HAVE_SLEEP which prevented
the substitution from being included in the source.  Adding a test for
_WIN32 (set by MinGW's gcc) then caused a warning from src/network.c on
POSIX about winsock2.h needing to be included before windows.h.  As
config.h needed to be included first, the solution to break out the
substitution that includes windows.h into its own file.  This patch
provides that solution and allows the code to compile cleanly on POSIX,
using MinGW on both POSIX and Windows, and on Cygwin.