md5 & metalink: use better build macros on Apple operating systems

Previously we used __MAC_10_X and __IPHONE_X to mark digest-generating
code that was specific to OS X and iOS. Now we use
__MAC_OS_X_VERSION_MAX_ALLOWED and __IPHONE_OS_VERSION_MAX_ALLOWED
instead of those macros.

Bug: http://sourceforge.net/p/curl/bugs/1255/
Reported by: Edward Rudd
darwinssl: fix build error in crypto authentication under Snow Leopard

It turns out Snow Leopard not only has SecItemCopyMatching() defined in
a header not included by the omnibus header, but it won't work for our
purposes, because searching for SecIdentityRef objects wasn't added
to that API until Lion. So we now use the old SecKeychainSearch API
instead if the user is building under, or running under, Snow Leopard.

Bug: http://sourceforge.net/p/curl/bugs/1255/
Reported by: Edward Rudd
darwinssl: added LFs to some strings passed into infof()

(This doesn't need to appear in the release notes.) I noticed a few places
where infof() was called, and there should've been an LF at the end of the
string, but there wasn't.