[CMake] Trouble with FindPNG module

[Robert Dailey]

I'm running CMake 3.8.0 on Ubuntu 14. I invoke the following:

find_package(PNG REQUIRED)

Which gives me the output in CMake:

Could NOT find PNG (missing: PNG_LIBRARY) (found version "1.2.50")

The CMakeCache.txt file has these variables set:

PNG_LIBRARY_DEBUG:FILEPATH=PNG_LIBRARY_DEBUG-NOTFOUND
PNG_LIBRARY_RELEASE:FILEPATH=PNG_LIBRARY_RELEASE-NOTFOUND
PNG_PNG_INCLUDE_DIR:PATH=/usr/include

So it found the headers, but not the libs. Why did it not find the
libs? Note that my version of Ubuntu is 64-bit, and I've installed the
32-bit libs like so:

$ sudo apt-get install libpng12-dev:i386

Would the find module be confused because it is trying to find the
64-bit library? What's the issue?