Configure: avoid perl regexp bugs

Configure: avoid perl regexp bugs

It seems that in older perl versions '(?P' doesn't interact very well
with '(?|' or '(?:'.

Since we make extensive use of '(?P' in build.info parsing, we avoid
combining that with '(?|' and '(?:' when parsing build.info variables,
and end up parsing variable modifier twice (first generally, and then
parse that result into the modifier components).

Fixes #11694

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/11737)