bug#23599: [PATCH 2/2] texi: define new AM_TEXI2FLAGS variable

[Karl Berry]

I don't really understand why this patch is in two parts, with seemingly
the same change, but whatever, doesn't matter.
    
    * doc/automake.texi: Mention new AM_TEXI2FLAGS setting.
    * lib/am/texibuild.am: Pass $(AM_TEXI2FLAGS) to TEXI2DVI & TEXI2PDF.

1) Maybe something should be said about the -I propagation/restriction?
I have to admit I don't see where in your changes the -I comes into
play, other than being stated in the comments.

2) Needs NEWS entry. --thanks, karl.