Use static linking for libarchive by default on MacOS

Use static linking for libarchive by default on MacOS

Apparently dlopen() path searching doesn't work or is unreliable on
MacOS; it appears to not search LD_LIBRARY_PATH, DYLD_LIBRARY_PATH, or
DYLD_FALLBACK_LIBRARY_PATH as documented by the dlopen(3) man page.
Furthmore setting any of these variables to include /opt/local/lib,
which is where MacPort drops libarchive.13.dylib, can cause all manner
of warnings when running other dynamically linked programs, such as
"grep".  What a mess.  It appears dlopen(2) support in MacOS is a
disaster, and it's not really needed since we don't care about the
size of mke2fs on installation media on non-Linux systems.  So default
to using --with-libarchive=direct for MacOS.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>