Fix some MinGW build issues...

mpm_winnt.c:  MinGW doesn't currently define the Windows
              STACK_SIZE_PARAM_IS_A_RESERVATION symbol

mpm_unix.c:   Bypass all this code on Windows  (too much trouble
              to keep it out of Makefile)

mpm_common.h: Skip over definitions of functions not available on
              Windows to keep references out of exports.c.

PR: 49535
Submitted by: John Vandenberg <jayvdb gmail.com>
Minor tweaks by: trawick

Other commits for this PR: r1089950, r1089951, r1089954


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1090614 13f79535-47bb-0310-9956-ffa450edef68