libmat2: fix dependency checks for cmd line utilities

The command line checks for command line utilities are done by trying to
access the executables and by throwing an exception when not found. This
lead to:
- The mat2 cmd line --check-dependencies option failing.
- The ffmpeg unit tests failing when ffmpeg isn't installed (even though
  it's an optional dependency).

This patch fixes it.

Signed-off-by: Antoine Tenart <antoine.tenart@ack.tf>