- [libpng] set CMAKE_SYSTEM_PROCESSOR for M1

[SSE4]

see https://www.gitmemory.com/issue/glennrp/libpng/372/808830443

Specify library name and version: libpng/all

This is also a good place to share with all of us why you are submitting this PR (specially if it is a new addition to ConanCenter): is it a dependency of other libraries you want to package? Are you the author of the library? Thanks!

---

• I've read the guidelines for contributing.
• I've followed the PEP8 style guides for Python code in the recipes.
• I've used the latest Conan client version.
• I've tried at least one configuration locally with the
  conan-center hook activated.

[conan-center-bot]

All green in build 2 (1ba6329d42719e1c0ee5c027d2ebf077456fe1c9):

• libpng/1.6.37@:
  All packages built successfully! (All logs)

[SpaceIm]

It fails also for iOS, but I don't know which CMAKE_SYSTEM_PROCESSOR to set.
ok: #2366, maybe this recipe should use autotools for non-Windows platform

[jgsogo]

Well, in fact, this is only required when cross-building (because we are not using toolchain file)

[SSE4]

iOS has different architectures (armv7, armv8 and their variants), so CMAKE_SYSTEM_PROCESSOR will have different values

[jgsogo]

Sure, my suggestion is to force this value only for the cross-building scenario, otherwise, in a native build I would expect it to work out of the box. wdyt?

[SSE4]

as I understand, in native builds CMAKE_SYSTEM_PROCESSOR defaults to CMAKE_HOST_SYSTEM_PROCESSOR which should be auto-detected to aarch64 by cmake itself? still, I don't have ARM M1 laptop to check it, so no idea if it works without explicit definition...

[SpaceIm]

It works for iOS:

        if tools.is_apple_os(self.settings.os):
            if "arm" in self.settings.arch:
                cmake.definitions["PNG_ARM_NEON"] = "on" if self.settings.os == "Macos" else "off"
            if self.settings.arch == "armv8":
                cmake.definitions["CMAKE_SYSTEM_PROCESSOR"] = "aarch64"

I would expect conan to handle this automatically (except PNG_ARM_NEON of course, here we should switch to autotools, it seems to properly handle neon for iOS).