error in meson build when using custom libpng build

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Hello team,

I've found myself in a situation where I was unable to build freetype 
using meson. The issue seems to be that I was using a variant of libpng 
locally built with CMake. As it turns out, with the current build script 
meson expects certain files to be named libpngconfig.cmake for example 
but libpng installs them as pngconfig.cmake. Meson is hence not able to 
recognize them and falls back to the subproject wrap file which doesn't 
have that issue. However, this does not work for me because I forbid 
downloading additional packages.

To fix this, I changed the meson build file to include a fallback 'png' 
library name in addition to the existing 'libpng' (new feature as of 
meson v0.60.0). With that, I was successfully able to build freetype.

I'd like to contribute my fix to freetype, please find a patch file 
attached. If you have any question regarding my patch and/or the problem 
I'm trying to address, please let me know.

Regards
lucebac
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