The `which` command is not part of the posix standard and not available
in some environments. The `command` command is part of the posix
standard and well supported.
See https://unix.stackexchange.com/q/85249 for a discussion about the
use of `command` instead of `which`.
If a system had `libtool` but not `which`, the build process would issue
an erroneous error stating:
libtool is required, but wasn't found on this system
Switching to `command` corrects this problem.
