unified build scheme: add the tweaks to build on Cygwin & Mingw

Cygwin and Mingw name their libraries a bit differently from the rest
of the POSIXly universe, we need to adapt to that.

In Makefile.tmpl, it means that some hunks will only be output
conditionally.

This also means that shared_extension for the Cygwin and Mingw
configurations in Configurations/10-main.conf are changing from .dll.a
to .dll.  Makefile.shared does a fine job without having them
specified, and it's much easier to work with tucking an extra .a at
the end of files in the installation recipes than any amount of name
rewrites, especially with the support of the SHARED_NAME in the top
build.info.

Reviewed-by: Rich Salz <rsalz@openssl.org>