Webalizer via brew, trying to build with libpng15

Question

I updated brew sometime ago and recently tried to run webalizer and now get this error:

libpng warning: Application built with libpng-1.2.41 but running with 1.5.10 gd-png error: cannot allocate libpng main struct

I ran libpng-config --cflags --ldflags to get the CFLAGS and LDFLAGS variables, and added them to the info file:

def install
  ENV['CFLAGS']='-I/opt/X11/include/libpng15'
  ENV['LDFLAGS']='-L/opt/X11/lib -lpng15'
  system "./configure", "--disable-debug",
                       "--disable-dependency-tracking",
                       "--prefix=#{prefix}"
  system "make install"
end

brew builds from this info file okay, but I still have the libpng error.

How can I build webalizer so it's using the right libpng library?