While 325b3042edd6 fixed it on MINIX, it broke it on systems
that output the word "version" on several lines with "cc -v".
The fix is to only consider "clang version" or "LLVM version"
as clang version, but this time only using sed(1).

As per POSIX, basic regular expressions have no alternations, and the
interpretation of the "\|" construct is undefined.  At least on MINIX
and Solaris grep interprets "\|" as literal "|", and not as an alternation
as GNU grep does.  Removed such constructs introduced in f1daa0356a1d.
This fixes clang detection on MINIX.

This reduces the number of moving parts in ABI compatibility checks.
Additionally, it also allows to use OpenSSL in FIPS mode while still
using md5 for non-security tasks.

Previous workaround to avoid warnings on OS X due to deprecated system
OpenSSL library (introduced in a3870ea96ccd) no longer works, as
the MAC_OS_X_VERSION_MIN_REQUIRED macro is ignored on OS X 10.9
if a compiler used supports __attribute__(availability).

Modern clang versions seem to no longer produce warnings for
system headers on Linux (at least clang 3.3 works), hence the
change.  For older versions --with-cc-opt="-Wno-error" can be
used as a workaround.

The -Werror is commented out to not break builds on Linux.