Fix default installation paths on mingw

Mingw config targets assumed that resulting programs and libraries are
installed in a Unix-like environment and the default installation
prefix was therefore set to '/usr/local'.

However, mingw programs are installed in a Windows environment, and
the installation directories should therefore have Windows defaults,
i.e. the same kind of defaults as the VC config targets.

A difficulty is, however, that a "cross compiled" build can't figure
out the system defaults from environment the same way it's done when
building "natively", so we have to fall back to hard coded defaults in
that case.

Tests can still be performed when cross compiled on a non-Windows
platform, since all tests only depend on the source and build
directory, and otherwise relies on normal local paths.

CVE-2019-1552

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/9400)