Just for the records, this is how the build fails:
...
libtool: compile: gcc -DHAVE_CONFIG_H -I. -I../../.. -I../../../src
-I../../../src/import-export -I../../../src/gnome
-I../../../src/register/ledger-core
-I../../../src/register/register-gnome
-I../../../src/register/register-core -I../../../src/gnome-utils
-I../../../src/app-utils -I../../../src/engine -I../../../src/core-utils
-I../../../src/gnc-module -I../../../src/libqof/qof -pthread
-I/usr/include/gtk-2.0 -I/usr/lib/x86_64-linux-gnu/gtk-2.0/include
-I/usr/include/gio-unix-2.0/ -I/usr/include/cairo
-I/usr/include/pango-1.0 -I/usr/include/atk-1.0 -I/usr/include/cairo
-I/usr/include/pixman-1 -I/usr/include/libpng16
-I/usr/include/gdk-pixbuf-2.0 -I/usr/include/libpng16
-I/usr/include/pango-1.0 -I/usr/include/harfbuzz
-I/usr/include/pango-1.0 -I/usr/include/glib-2.0
-I/usr/lib/x86_64-linux-gnu/glib-2.0/include -I/usr/include/freetype2
-pthread -I/usr/include/glib-2.0
-I/usr/lib/x86_64-linux-gnu/glib-2.0/include -I/usr/include/aqbanking5
-I/usr/include/gwenhywfar4 -DG_LOG_DOMAIN=\"gnc.import.aqbanking\"
-Wdate-time -D_FORTIFY_SOURCE=2 -Wdeclaration-after-statement
-Wno-pointer-sign -D_FORTIFY_SOURCE=2 -g -O2 -fstack-protector-strong
-Wformat -Werror=format-security -std=gnu99 -Wall -Wunused
-Wmissing-prototypes -Wmissing-declarations -Wno-unused -c
gnc-gwen-gui.c -fPIC -DPIC -o .libs/gnc-gwen-gui.o
gnc-gwen-gui.c:64:37: fatal error: gwen-gui-gtk2/gtk2_gui.h: No such
file or directory
compilation terminated.
Makefile:716: recipe for target 'gnc-gwen-gui.lo' failed
make[6]: *** [gnc-gwen-gui.lo] Error 1
make[6]: Leaving directory
'/tmp/buildd/gnucash-2.6.12/src/import-export/aqb'
Makefile:773: recipe for target 'all-recursive' failed
make[5]: *** [all-recursive] Error 1
make[5]: Leaving directory
'/tmp/buildd/gnucash-2.6.12/src/import-export/aqb'
Makefile:758: recipe for target 'all-recursive' failed
make[4]: *** [all-recursive] Error 1
make[4]: Leaving directory '/tmp/buildd/gnucash-2.6.12/src/import-export'
Makefile:563: recipe for target 'all-recursive' failed
make[3]: *** [all-recursive] Error 1
make[3]: Leaving directory '/tmp/buildd/gnucash-2.6.12/src'
Makefile:796: recipe for target 'all-recursive' failed
make[2]: *** [all-recursive] Error 1
make[2]: Leaving directory '/tmp/buildd/gnucash-2.6.12'
Makefile:650: recipe for target 'all' failed
make[1]: *** [all] Error 2
make[1]: Leaving directory '/tmp/buildd/gnucash-2.6.12'
dh_auto_build: make -j1 returned exit code 2
debian/rules:15: recipe for target 'build' failed
make: *** [build] Error 2
dpkg-buildpackage: error: debian/rules build gave error exit status 2
E: Failed autobuilding of package
