Properly call out the requirements in the documentation.

1. Some testcases assume that the user has root privileges (use mdconfig
   -a / mount, etc).
2. Linux support exists at one degree or another.

Submitted as misc/161915.

Signed-off-by: Garrett Cooper <yanegomi@gmail.com>
Improve the Makefile

1. Add an all target
2. Conditionalize CC
3. Add LDLIBS/LDFLAGS

Signed-off-by: Garrett Cooper <yanegomi@gmail.com>
Fail quickly if mdconfig or newfs fails

Signed-off-by: Garrett Cooper <yanegomi@gmail.com>