matplotlib/matplotlib — pull request #3477 by cimarronm (2014-09-06 00:43:55+00:00)
Title: BUG: If pkgconfig returns nothing for libpng at least check for header f...

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.]
...
...

```

## Updated behavior
#### libpng pkgconfig exists
```
REQUIRED DEPENDENCIES AND EXTENSIONS
...
...
png: yes [version x.y.z]
...
...

```
#### 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.]
...
...

```
#### 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

```