install-from-source: avoid using which before it is installed

The `which` executable must often be installed because it is missing
from many a Docker image. Therefore, it won't _really_ work if one
checks `which which` to figure out whether `which` is installed.

Let's avoid this by using `type`, which is a shell builtin for most
shells.

The `type` utility is specified in the POSIX standard, as per
https://pubs.opengroup.org/onlinepubs/9699919799/utilities/type.html,
yet neither command-line options nor output is standardized. The only
thing we _can_ rely on is the exit status.

Note: _Technically_, this poses a change of behavior, as `which`
resolves only to executables that are on the `PATH` while `type` will
also happily report shell builtins. However, this is a net improvement:
If running the script in, say, BusyBox, where many of the common
utilities (including `which`!) are shell builtins, we would like to
avoid forcefully installing the packages without need.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

src/linux/Packaging.Linux/install-from-source.sh

@@ -63,7 +63,7 @@ install_packages() {
 
     for package in $packages; do
         # Ensure we don't stomp on existing installations.
-        if [ ! -z $(which $package) ]; then
+        if type $package >/dev/null 2>&1; then
             continue
         fi