Fix md5.c++ to work on non x86 systems

ctx is not a pointer but was being used with the arrow operator on non-x86 systems
Merge pull request #34 from dschatzberg/patch-1

Fix md5.c++ to work on non x86 systems
fix output of Int8 values for Solaris
make capnp-test.sh compatible with Solaris sh(1), test(1) and cmp(1)
Solaris doesn't have backtrace()