Link to Pthread-win32 library in MinGW

The AX_PTHREAD macro sets the PTHREAD_LIBS variable, however we were not
providing this variable to the needed *_LDFLAGS variable.  With the
addition of PTHREAD_LIBS, rigctld and rotctld are now multi-threaded on
MS Windows.  Tested on Windows 2000, Windows XP, and Windows 7.
Fix W2k 'freeaddrinfo' error

Trying to run the pthread enabled binaries on W2k resulted in an error
dialog with the text, "The procedure entry point freeaddrinfo could not be
located in ws2_32.dll".  A Microsoft support page
(http://support.microsoft.com/kb/955045) hints that when 'ws2tcpip.h' is
included that 'wspiapi.h' should be included as well.  Since MinGw
includes both files, this patch corrects the runtime error on W2k