header location for png.h should be used in the build

[Dmitrii Pasechnik]

In the settting such as Conda the compiler might not look in the usual locations for png.h, without given -I.

It should not be hard to pkg-config --cflags libpng (or the autoconf equivalent of this) to get the correct -I to add to CFLAGS. See https://groups.google.com/g/sage-devel/c/LMFRiG9Y0ZM/m/pq8gvU0TCQAJ for an m4ri failing build log

[Martin Albrecht]

add libpng cflags to various places

fixes #77

→ <<cset 689818bd24fa>>

[Martin Albrecht]

Can you test if this fixes the issue for you?

[Dmitrii Pasechnik]

In principle this should allow m4ri(e) to come to conda.

@Isuru Fernando

[Dmitrii Pasechnik]

yes, this builds and passes tests on the latest conda 4.9 (on Linux), with conda-supplied PNG.

[Dmitrii Pasechnik]

I’ve also checked that the m4rie built with so configured/built m4ri works, as well.