Better check for required dependency libpng

This fixes a bug where the setup incorrectly may state that libpng is present and continue building which will eventually error out.

## Current behavior
#### No libpng installed at all
```
REQUIRED DEPENDENCIES AND EXTENSIONS
...
...
                   png: yes [pkg-config information for 'libpng' could not
                        be found. Using unknown version.]
...
...
<builds until it eventually errors out>
```

## Updated behavior
#### libpng pkgconfig exists
```
REQUIRED DEPENDENCIES AND EXTENSIONS
...
...
                   png: yes [version x.y.z]
...
...
<builds>
```
#### no libpng pkgconfig bug png.h include exists
```
REQUIRED DEPENDENCIES AND EXTENSIONS
...
...
                   png: yes [pkg-config information for 'libpng' could not
                        be found. Using unknown version found on system.]
...
...
<builds>
```
#### no png.h include at all
```
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>
```