If you naively follow the steps on the GraphicsMagick website, after running ./configure, you end up with no PNG support:
checking for PNG support ...
checking png.h usability... no
checking png.h presence... no
PNG --with-png=yes no
This makes for a pretty useless GraphicsMagick instalation.
How can I enable PNG support? Libpng seems to be installed already, but not detected:
> $ sudo yum install libpng
> Package 2:libpng-1.2.49-1.12.amzn1.x86_64 already installed and latest version
You need to install the libpng-devel package, in addition to just libpng:
sudo yum install libpng-devel
Then when you run ./configure, everything will work out as you'd hope:
checking for PNG support ...
checking png.h usability... yes
checking png.h presence... yes
PNG --with-png=yes yes (-lpng12)
<Rant>This is why everything should be available in packages that clearly specify their dependencies and are easily installed with package managers, instead of requiring arcane knowledge of compilation prerequisites.</Rant>