config: Pass diverse flags to Configure via the environment

Passing flags "discovered" by 'config' on the command line to
'Configure' mixes them up with flags given by the user.  That is
contrary to their intention, so they need to be passed in a different
manner.

Enter the environment variables __CNF_CPPDEFINES, __CNF_CPPINCLUDES,
__CNF_CPPFLAGS, __CNF_CFLAGS, __CNF_CXXFLAGS, __CNF_LDFLAGS, and
__CNF_LDLIBS, initialised by 'config', and then used by Configure to
initialise these %config values: defines, includes, cppflags, cflags,
cxxflags, lflags, and ex_libs.

Reviewed-by: Andy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5534)