REQUIRED DEPENDENCIES AND EXTENSIONS
...
...
                   png: yes [pkg-config information for 'libpng' could not
                        be found. Using unknown version.]
...
...
<builds until it eventually errors out>

REQUIRED DEPENDENCIES AND EXTENSIONS
...
...
                   png: yes [version x.y.z]
...
...
<builds>

REQUIRED DEPENDENCIES AND EXTENSIONS
...
...
                   png: yes [pkg-config information for 'libpng' could not
                        be found. Using unknown version found on system.]
...
...
<builds>

REQUIRED DEPENDENCIES AND EXTENSIONS
...
...
                   png: no  [pkg-config information for 'libpng' could not
                        be found.]
...
...
============================================================================
                        * The following required packages can not be built:
                        * png
<does not build>