mlog: Respect preexisting $LESS for pager

[meator]

I grew tired of seeing the following every time I wanted to check options with meson configure:

I have LESS set to --ignore-case, which also coicidentally disabled Meson's -R.

[bonzini]

Meson's current behavior is, at least, consistent with Git. From git-config(1):

When the LESS environment variable is unset, Git sets it to FRX (if LESS environment variable is set, Git does not change it at all).

[meator]

I just checked and it's broken on git too. I usually use a custom pager for git, so I did not notice.

TBH I don't agree with git. I just want to set --ignore-case, is that a good enough reason for meson and git to produce garbled output? I don't want to set the other stuff permanently too. And I think this solution is better than having to change global LESS or having to add an alias or custom function to override LESS for that one command.

I'll undetstand if meson wishes to keep following git behavior.