Tue May 3 15:05:30 PDT 2016
"SDL2 has dropped support for PowerPC Macs and OS X versions older
than 10.5 (SDL 1.2 still supports PPC and 10.0, though)."
Which implies that 10.5 is still supported, and indeed they describe
the Mac binary download as "Intel 10.5+"
Libsdl2 not building on 10.6 is either a bug in MacPorts, a bug in
libsdl2 or a bug in libsdl's documentation.
More information about the macports-users