Building emacs errors: libraries present

[Tory S. Anderson]

Related but distinct from another post (other post has 24.4 failing at the make 
stage, but 25.0.5 fails at configure):

I am unable to build emacs with png support, despite my having the libpng and 
libpng-devel libraries. Why might this be? I'm running Linux Fedora 20.

(abridged) Output of my attempts to configure:
==============================
$ ./configure
... # list omitted
configure: error: The following required libraries were not found:
     libpng libgif/libungif
Maybe some development libraries/packages are missing?
If you don't want to link with them give
     --with-png=no --with-gif=no
as options to configure

$ sudo yum install libpng libpng-devel
... # list omitted
Package 2:libpng-1.6.6-3.fc20.x86_64 already installed and latest version
Package 2:libpng-devel-1.6.6-3.fc20.x86_64 already installed and latest version
Nothing to do

$ whereis libpng
libpng: /usr/lib64/libpng16.so /usr/lib64/libpng.so 
/usr/share/man/man3/libpng.3.gz
==============================

Of course I've successfully built without the png libraries but I really do 
need the png libraries for other emacs packages. What's wrong? Why is it 
ignoring my working libpng?