Consider enabling MD5 on Mac

[Peter Hyman]

In a 2011 ChangeLog entry:

APRIL 2011, version 0.602 Con Kolivas
. . .

• Disable md5 generation and checking on Apple till it's fixed.

My testing shows that MD5 computes properly now (at least using libgcrypt). MD5 should be the default and the #define MD5_RELIABLE should not be checked for. See lzma-21.03beta branch.

[Con Kolivas]

Enabled.