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