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.
