Since the build.sh script runs with `set -e` (exit _immediately_ in case
of error), we cannot first call the `which` command and, on a susequent
line, check its exit status with $?, it would be too late. Instead, we
idiomatically check on the same line of the invocation of `which`.
From the confusing:
$ make bin
==> Checking for necessary tools...
make: *** [bin] Error 1
To the informative:
$ make bin
==> Checking for necessary tools...
realpath is not on the path. Exiting...
make: *** [bin] Error 1
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