Log Message:
Allow packages that use OpenSSL to specify that they need an OpenSSL
built with support for threads.  This is done by adding the following
line to the package Makefile before the inclusion of openssl/buildlink3.mk:

    USE_FEATURES.openssl=	threads

The openssl/builtin.mk file is also adjusted to detect whether or not
the built-in OpenSSL was built with support for threads and the result
is used accordingly to determine whether or not a pkgsrc OpenSSL is
needed.