Configure: base compiler-specific decisions on pre-defines.

The commit subject is a bit misleading in sense that decisions affect
only gcc and gcc-alikes, like clang, recent icc...

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Ben Kaduk <kaduk@mit.edu>
(Merged from https://github.com/openssl/openssl/pull/4281)