With CFLAGS set as in 7da71a7b141a, OpenSSL compilation drops various
non-important compiler options.  To avoid this, a define is added
instead - OpenSSL is smart enough to recognize -D... in Configure
arguments.

By default, MSVC uses vc<version>.pdb in the current directory.
With the "-Fd" switch it is directed to be in the objs directory instead.

OpenSSL 1.1.0 and above uses BCrypt if available (Windows 7 or higher).
This results in an unusable binary on older Windows versions, when building
with newer Windows SDK (such as 7.0A).  Using CFLAGS to define _WIN32_WINNT
allows to set a desired ABI and make sure the binary works with Windows XP.

To not mix with other potential CFLAGS uses, it is set in GNUmakefile.

When building OpenSSL 1.1.1b, as used for win32 builds, with tests
it takes about twice as long and near ~1GB of additional disk space.
Using "no-tests" OpenSSL configuration option allows to skip them.
Since such an option is supported since OpenSSL 1.1.1 only, it is
residing here and not in configure.

OCSP response uses the DER format and as such needs to be opened in binary-mode.
This only has any effect under Win32.

If X509_get_issuer_name() or X509_get_subject_name() returned NULL,
this could lead to a certificate reference leak.  It cannot happen
in practice though, since each function returns an internal pointer
to a mandatory subfield of the certificate successfully decoded by
d2i_X509() during certificate message processing (closes #1751).