Auto merge of #22169 - gfxmonk:disable-docs, r=steveklabnik

Auto merge of #22169 - gfxmonk:disable-docs, r=steveklabnik

`make docs` fails when (xe)latex is not installed. The output is pretty weird, looks like it's doing some `eval` tricks but something is blank:

    /bin/sh: -output-directory=.: command not found
    /home/tim/dev/rust/rust/mk/docs.mk:220: recipe for target 'doc/reference.pdf' failed

I have neither latex or xelatex installed. It seems like the following snippet is meant for me, but the logic is backwards:

    ifeq ($(CFG_XELATEX),)
        CFG_LATEX := $(CFG_XELATEX)
        XELATEX = 1
      else
        $(info cfg: no xelatex found, disabling LaTeX docs)
        NO_PDF_DOCS = 1
    endif

I verified with:

    $ make doc/reference.pdf CFG_XELATEX=/bin/foo
    cfg: no xelatex found, disabling LaTeX docs