rtla/Makefile: Properly handle dependencies

Linus had a problem compiling RTLA, saying:

"[...] I wish the tracing tools would do a bit more package
checking and helpful error messages too, rather than just
fail with:

fatal error: tracefs.h: No such file or directory"

Which is indeed not a helpful message. Update the Makefile, adding
proper checks for the dependencies, with useful information about
how to resolve possible problems.

For example, the previous error is now reported as:

$ make
********************************************
** NOTICE: libtracefs version 1.3 or higher not found
**
** Consider installing the latest libtracefs from your
** distribution, e.g., 'dnf install libtracefs' on Fedora,
** or from source:
**
** https://git.kernel.org/pub/scm/libs/libtrace/libtracefs.git/
**
********************************************

These messages are inspired by the ones used on trace-cmd, as suggested
by Stevel Rostedt.

Link: https://lore.kernel.org/r/CAHk-=whxmA86E=csNv76DuxX_wYsg8mW15oUs3XTabu2Yc80yw@mail.gmail.com/

Changes from V1:
- Moved the rst2man check to the install phase (when it is used).
- Removed the procps-ng lib check [1] as it is being removed.

[1] a0f9f8c1030c66305c9b921057c3d483064d5529.1651220820.git.bristot@kernel.org

Link: https://lkml.kernel.org/r/3f1fac776c37e4b67c876a94e5a0e45ed022ff3d.1651238057.git.bristot@kernel.org

Cc: Ingo Molnar <mingo@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Daniel Bristot de Oliveira <bristot@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

rtla: Add Documentation

Adds the basis for rtla documentation. This patch also
includes the rtla(1) man page.

As suggested by Jonathan Corbet, we are placing these man
pages at Documentation/tools/rtla, using rst format. It
is not linked to the official documentation, though.

The Makefile is based on bpftool's Documentation one.

Link: https://lkml.kernel.org/r/5f510f3e962fc0cd531c43f5a815544dd720c3f2.1639158831.git.bristot@kernel.org

Cc: Tao Zhou <tao.zhou@linux.dev>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Tom Zanussi <zanussi@kernel.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Clark Williams <williams@redhat.com>
Cc: John Kacur <jkacur@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Daniel Bristot de Oliveira <bristot@kernel.org>
Cc: linux-rt-users@vger.kernel.org
Cc: linux-trace-devel@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Suggested-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Daniel Bristot de Oliveira <bristot@kernel.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>