Bug#1140847: PATH_MAX not defined on hurd

[Drew Parsons]

Package: cimg-dev
Version: 3.5.2+dfsg-2
Severity: normal
Control: affects -1 src:jkqtplotter

/usr/include/CImg.h uses PATH_MAX, which is defined on unix but not on
Hurd.

The issue has a bit of history,
https://www.gnu.org/software/hurd/hurd/porting/guidelines.html
https://www.gnu.org/software/hurd/user/tlecarrour/porting_guide_for_dummies.html
https://www.gnu.org/software/hurd/community/gsoc/project_ideas/maxpath.html
http://insanecoding.blogspot.com/2007/11/pathmax-simply-isnt.html
https://lists.debian.org/debian-hurd/2025/01/msg00013.html
https://reviews.llvm.org/D53410

The simplest workaround is to just define it (e.g. to 1024 or 4096) in CImg.h
if it's not already defined.

It's currently affecting jkqtplotter,
https://buildd.debian.org/status/fetch.php?pkg=jkqtplotter&arch=hurd-i386&ver=5.0.0%7Ebeta2%2Bgit20260626.e82cd41%2Bdfsg1-1&stamp=1782580500&raw=0