Hello,

This patch adds basic Texinfo support for Flymake, following the
annotated example in the Flymake manual.  All it does is feed the
contents of the buffer to the program `makeinfo`, which either reports
errors or outputs the resulting file to the specified /dev/null.

The regexp for identifying errors and warnings is the same as used by
the package Flycheck (see here:
https://github.com/flycheck/flycheck/blob/784f184cdd9f9cb4e3dbb997c09d93e954142842/flycheck.el#L12175).
It seems to identify an optional column number, but I didn't see that in
the programs output or listed in the Man page.  The patch does not make
use of any match for that information.

The Man page says that the program will by default report 100 errors
before stopping.

While the package Flymake is required during compilation, the byte
compiler still wanted the function `flymake--log-1` declared.  This is
the function run by the macro `flymake-log`.

Thank you.