Bring in basic support for building the winnt MPM under mingw32, using the autoconf...

Bring in basic support for building the winnt MPM under mingw32, using the autoconf build system.  Currently this will get you to the point of linking errors on gen_test_char.

configure.in: Default to winnt MPM on mingw32 hosts.
configure.in: Add AP_DECLARE_EXPORT for mingw32 hsots.

os/config.m4: Use win32 dir under mingw32.
os/win32/Makefile.in: Add in a new makefile to generate libos on win32.

server/mpm/config.m4: Add winnt MPM to the help string.
server/mpm/config.m4: winnt MPM is a threaded MPM.

server/mpm/winnt/child.c: SO_UPDATE_ACCEPT_CONTEXT is defined under a different header in mingw32.

server/mpm/winnt/{Makefile.in,config.m4}: Base build for winnt MPM.

os/win32,server/mpm/winnt: Update svn:ignore for .deps/.libs

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@422182 13f79535-47bb-0310-9956-ffa450edef68