+ shlib_variant => A "variant" identifier inserted between the base
+ shared library name and the extension. On "unixy"
+ platforms (BSD, Linux, Solaris, MacOS/X, ...) this
+ supports installation of custom OpenSSL libraries
+ that don't conflict with other builds of OpenSSL
+ installed on the system. The variant identifier
+ becomes part of the SONAME of the library and also
+ any symbol versions (symbol versions are not used or
+ needed with MacOS/X). For example, on a system
+ where a default build would normally create the SSL
+ shared library as 'libssl.so -> libssl.so.1.1' with
+ the value of the symlink as the SONAME, a target
+ definition that sets 'shlib_variant => "-abc"' will
+ create 'libssl.so -> libssl-abc.so.1.1', again with
+ an SONAME equal to the value of the symlink. The
+ symbol versions associated with the variant library
+ would then be 'OPENSSL_ABC_<version>' rather than
+ the default 'OPENSSL_<version>'. The string inserted
+ into symbol versions is obtained by mapping all
+ letters in the "variant" identifier to upper case
+ and all non-alphanumeric characters to '_'.
+
+ thread_scheme => The type of threads is used on the
+ configured platform. Currently known
+ values are "(unknown)", "pthreads",
+ "uithreads" (a.k.a solaris threads) and
+ "winthreads". Except for "(unknown)", the
+ actual value is currently ignored but may
+ be used in the future. See further notes
+ below [2].