fix configure errors for 64bit mingw

changes the search pattern for host_os from *mingw32* to *mingw* so that mingw64 can be used, and any mingw32 variable name change to mingw.

acinclude.m4

@@ -117,7 +117,7 @@ AC_DEFUN([LIBZMQ_CHECK_DOC_BUILD], [{
         fi
 
         # Do not install man pages if on mingw
-        if test "x$libzmq_on_mingw32" = "xyes"; then
+        if test "x$libzmq_on_mingw" = "xyes"; then
             libzmq_install_man="no"
         fi
     fi