util/perl/OpenSSL/config.pm: Fix determine_compiler_settings()

util/perl/OpenSSL/config.pm: Fix determine_compiler_settings()

There may be times when a compiler can't be detected, in which case
determine_compiler_settings() bailed out too early, before platform
specific fallbacks have a chance to set the record straight.  That
bail out has been moved to be done after the platform specific
fallbacks.

Furthermore, the attempt to check for gcc or clang and get their
version number was done even if no compiler had been automatically
detected or pre-specified via $CC.  It now only does this when there
is a compiler specified or detected.  The platform specific fallbacks
check the versions separately.

Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/14270)