Use shared library symlink instead of header for testing ncurses.

[delphij]

Summary

The current code uses the header when testing ncurses. On portsnap
builder, the header is not available at this time.

Note that /lib/libncurses.so have gone long ago and 9, 10, 11 all have
/usr/lib/libncurses.so, I think it's reasonable to just use the shared
library itself to determine the availability.

[delphij]

Note: this is no longer required for a correct portsnap build as I fixed the stored image, but I think this should still be considered for the sake of consistency.