use portable "command -v" to detect installed programs, part 4

The "which" utility is not guaranteed to be installed either, and if it
is, its behavior is not portable either. This means that when various
programs are installed, the `which` check will report a fatal error
because the which tool did not exist and the shell returned a nonzero
status when attempting to fork+exec. If it did exist, it might not be an
implementation of `which` that returns nonzero when commands do not
exist.

The general scripting suggestion is to use the "command -v" shell
builtin; this is required to exist in all POSIX 2008 compliant shells,
and is thus guaranteed to work everywhere.

For some in-depth discussions on the topic, see:
- https://mywiki.wooledge.org/BashFAQ/081
- https://unix.stackexchange.com/questions/85249/why-not-use-which-what-to-use-then/85250#85250

Examples of open-source shells likely to be installed as /bin/sh on
Linux, which implement the 15-year-old standard: ash, bash, busybox,
dash, ksh, mksh and zsh.

This commit updates a couple build scripts to not rely on fixed paths
calculated upfront. Checking the location of a tool with cmd=$(which
foo) just to run it as `$cmd` is pointless. It's exactly equivalent to
run it as `foo`, but less error-prone and easier to read.

For one of the scripts, it also simplifies checking for their existence.
Personally, I am skeptical it even makes sense to check at all. POSIX
mandates they exist, and it's exceedingly unlikely they will not be
installed. However, unlike the shell interpreter itself, we don't *know*
they are installed, so leave the existing checks in but simplified.

Change-Id: I4703c1165eb3a5aeb45fbab18df3222e8f5f9551
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/160665
Tested-by: Jenkins
Tested-by: Ilmari Lauhakangas <ilmari.lauhakangas@libreoffice.org>
Reviewed-by: Ilmari Lauhakangas <ilmari.lauhakangas@libreoffice.org>

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