[gentoo-user] trouble merging gnucash (png16 vs png15)

One of my machines (amd64) cannot merge gnucash-2.6.1.
The complaint is that it can't find libpng15.  The system has libpng16.

The build log has 352 occurrences of libpng16 and no occurrences of
libpng15.

The build log has 2 occurrences of lpng both 15 in consecutive lines.
 libtool: link: [big snip] -lpng15 [big snip]
 [small snip] cannot find -lpng15

I seem to remember a few years ago having to edit pngXX to pngYY, but
thought those times were definitely over.

Any help would be appreciated.
allan

The full two lines with lpng are

libtool: link: x86_64-pc-linux-gnu-gcc -shared  -fPIC -DPIC  .libs/gncmod-register-gnome.o .libs/combocell-gnome.o .libs/datecell-gnome.o .libs/formulacell-gnome.o .libs/gnucash-color.o .libs/gnucash-cursor.o .libs/gnucash-date-picker.o .libs/gnucash-grid.o .libs/gnucash-header.o .libs/gnucash-item-edit.o .libs/gnucash-item-list.o .libs/gnucash-scrolled-window.o .libs/gnucash-sheet.o .libs/gnucash-style.o .libs/pricecell-gnome.o .libs/quickfillcell-gnome.o .libs/table-gnome.o   -Wl,-rpath -Wl,/var/tmp/portage/app-office/gnucash-2.6.1/work/gnucash-2.6.1/src/register/register-core/.libs -Wl,-rpath -Wl,/var/tmp/portage/app-office/gnucash-2.6.1/work/gnucash-2.6.1/src/gnome-utils/.libs -Wl,-rpath -Wl,/var/tmp/portage/app-office/gnucash-2.6.1/work/gnucash-2.6.1/src/core-utils/.libs -Wl,-rpath -Wl,/var/tmp/portage/app-office/gnucash-2.6.1/work/gnucash-2.6.1/src/gnc-module/.libs -Wl,-rpath -Wl,/var/tmp/portage/app-office/gnucash-2.6.1/work/gnucash-2.6.1/src/engine/.libs -Wl,-rpath -Wl,/var/tmp/portage/app-office/gnucash-2.6.1/work/gnucash-2.6.1/src/app-utils/.libs -Wl,-rpath -Wl,/var/tmp/portage/app-office/gnucash-2.6.1/work/gnucash-2.6.1/src/backend/xml/.libs -Wl,-rpath -Wl,/var/tmp/portage/app-office/gnucash-2.6.1/work/gnucash-2.6.1/src/libqof/qof/.libs -Wl,-rpath -Wl,/usr/lib64/gnucash -L/var/tmp/portage/app-office/gnucash-2.6.1/work/gnucash-2.6.1/src/gnome-utils/.libs -L/var/tmp/portage/app-office/gnucash-2.6.1/work/gnucash-2.6.1/src/core-utils/.libs -L/var/tmp/portage/app-office/gnucash-2.6.1/work/gnucash-2.6.1/src/gnc-module/.libs -L/var/tmp/portage/app-office/gnucash-2.6.1/work/gnucash-2.6.1/src/engine/.libs -L/var/tmp/portage/app-office/gnucash-2.6.1/work/gnucash-2.6.1/src/app-utils/.libs -L/var/tmp/portage/app-office/gnucash-2.6.1/work/gnucash-2.6.1/src/backend/xml/.libs -L/var/tmp/portage/app-office/gnucash-2.6.1/work/gnucash-2.6.1/src/libqof/qof/.libs -Wl,--as-needed ../../../src/register/register-core/.libs/libgncmod-register-core.so -L/usr/lib64 /var/tmp/portage/app-office/gnucash-2.6.1/work/gnucash-2.6.1/src/gnome-utils/.libs/libgncmod-gnome-utils.so ../../../src/core-utils/.libs/libgnc-core-utils.so ../../../src/gnc-module/.libs/libgnc-module.so ../../../src/engine/.libs/libgncmod-engine.so ../../../src/app-utils/.libs/libgncmod-app-utils.so ../../../src/gnome-utils/.libs/libgncmod-gnome-utils.so /var/tmp/portage/app-office/gnucash-2.6.1/work/gnucash-2.6.1/src/backend/xml/.libs/libgnc-backend-xml-utils.so /var/tmp/portage/app-office/gnucash-2.6.1/work/gnucash-2.6.1/src/app-utils/.libs/libgncmod-app-utils.so /var/tmp/portage/app-office/gnucash-2.6.1/work/gnucash-2.6.1/src/engine/.libs/libgncmod-engine.so /var/tmp/portage/app-office/gnucash-2.6.1/work/gnucash-2.6.1/src/gnc-module/.libs/libgnc-module.so /var/tmp/portage/app-office/gnucash-2.6.1/work/gnucash-2.6.1/src/core-utils/.libs/libgnc-core-utils.so -lxslt /var/tmp/portage/app-office/gnucash-2.6.1/work/gnucash-2.6.1/src/libqof/qof/.libs/libgnc-qof.so -lgnome-keyring /usr/lib64/libguile.so -lgmp -lcrypt /usr/lib64/libltdl.so -lxml2 ../../../src/libqof/qof/.libs/libgnc-qof.so /usr/lib64/libgnomecanvas-2.so -L/usr/lib64/qt4 -lgailutil -lXi -lXrandr -lXcursor -lXcomposite -lXdamage -lXext -lXfixes -lxcb-render-util -lresolv -lpixman-1 -lpng15 -lxcb-shm -lxcb-render -lXrender -lX11 -lEGL -lQtGui -lQtCore -lxcb -lXau -lXdmcp -lGL -lgmodule-2.0 -lexpat -lz -lgthread-2.0 -ldl -lpthread -lrt /usr/lib64/libart_lgpl_2.so -lgtk-x11-2.0 -lgdk-x11-2.0 -lpangocairo-1.0 -latk-1.0 -lcairo -lgdk_pixbuf-2.0 -lgio-2.0 -lpangoft2-1.0 -lpango-1.0 -lgobject-2.0 -lglib-2.0 -lfreetype -lfontconfig -lm  -march=nocona -O2 -Wl,-O1 -pthread -Wl,--export-dynamic -pthread   -pthread -Wl,-soname -Wl,libgncmod-register-gnome.so -o .libs/libgncmod-register-gnome.so

/usr/lib/gcc/x86_64-pc-linux-gnu/4.7.3/../../../../x86_64-pc-linux-gnu/bin/ld: cannot find -lpng15