This is a problem with libpng but until it is changed you might want to make PHP aware of this issue and to compensate accordingly.

Under Tru64 UNIX (Digital UNIX/OSF) there is a slight problem in it finding the libpng library.

The solution is this:
cp /usr/local/lib/libpng.so.2.1.0.8 /usr/local/lib/libpng.so

You might want to put this in the make install since it could easily throw off someone and take a while for them to figure out.

The particular problem I was running into was compiling apache which included php4 which included gd which included libpng:
at-lite/libexpat.a  -Wl,-rpath,/usr/local/lib  -L/usr/local/lib -Lmodules/php4 -
L../modules/php4 -L../../modules/php4 -lmodphp4   -lpng -lz -lgd -lresolv -lm  -
lresolv   -lm
ld:
Unresolved:
png_set_sig_bytes
png_check_sig
png_create_read_struct
png_create_write_struct
png_create_info_struct
png_write_info
png_read_info
png_set_strip_alpha
png_set_packing
png_set_strip_16
png_set_dither
png_read_update_info
png_read_image
png_write_image
png_write_end
png_read_end
png_destroy_read_struct
png_destroy_write_struct
png_get_error_ptr
png_set_write_fn
png_set_read_fn
png_get_io_ptr
png_get_valid
png_get_rowbytes
png_get_IHDR
png_set_IHDR
png_get_PLTE
png_set_PLTE
png_get_tRNS
png_set_tRNS
*** Exit 1
Stop.
*** Exit 1
Stop.
*** Exit 1
Stop.