ISIS initialization error

Upon trying to initialize ISIS, I am receiving the following error:

--------------

*** isis initialization failed; using ISIS_SRCDIR = /usr/local/isis

Error linking to /usr/local/isis/lib/modules/pgplot-module.so: 
/usr/lib/libpgplot.so.5: undefined symbol: png_set_text
Traceback: import
/usr/local/isis/share/pgplot.sl:1:<top-level>:Import Error
Traceback: evalfile
Traceback: evalfile
*** isis initialization failed

--------------

I have tried every combination of compiling pgplot I can think of and 
have followed every tutorial I've found.  Have confirmed that that 
symbol does exist:

$ nm /usr/lib/libpng.a | grep png_set_text
000016e0 T png_set_text
00000f50 T png_set_text_2
         U png_set_text_2
nm: libpng12_la-pnggccrd.o: no symbols
         U png_set_text_2


Any help is greatly appreciated,
Keith McDermott
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