Comment 2 for bug 1802674

Well, I did find a working fix to apply in debian/rules.

# Add missing math libraray for linking in gphoto2
# (automatic math library linking was dropped by GCC years ago)
export DEB_LDFLAGS_MAINT_APPEND := -lm

I did add as well all available processor cores in parallel build, and Optimization O3 instead of -O2.

However it would better if upstream would update its own configure.ac file ...

Here is the portion of the log showing no sign of unresolved dependencies:

make[1]: Leaving directory '/home/ubuntu-xenial/pascal/Downloads/Launchpad/libgphoto2/libgphoto2-2.5.9'
   debian/rules override_dh_makeshlibs
make[1]: Entering directory '/home/ubuntu-xenial/pascal/Downloads/Launchpad/libgphoto2/libgphoto2-2.5.9'
dh_makeshlibs -plibgphoto2-6 -X/usr/lib/x86_64-linux-gnu/libgphoto2/
dh_makeshlibs -plibgphoto2-port12 -X/usr/lib/x86_64-linux-gnu/libgphoto2_port/
make[1]: Leaving directory '/home/ubuntu-xenial/pascal/Downloads/Launchpad/libgphoto2/libgphoto2-2.5.9'
   debian/rules override_dh_shlibdeps
make[1]: Entering directory '/home/ubuntu-xenial/pascal/Downloads/Launchpad/libgphoto2/libgphoto2-2.5.9'
dh_shlibdeps -ldebian/libgphoto2-6/usr/lib/:debian/libgphoto2-port12/usr/lib/
make[1]: Leaving directory '/home/ubuntu-xenial/pascal/Downloads/Launchpad/libgphoto2/libgphoto2-2.5.9'
   dh_installdeb -O--parallel
   debian/rules override_dh_gencontrol
make[1]: Entering directory '/home/ubuntu-xenial/pascal/Downloads/Launchpad/libgphoto2/libgphoto2-2.5.9'
dh_gencontrol
dpkg-gencontrol: warning: Depends field of package libgphoto2-port12-dbgsym: unknown substitution variable ${shlibs:Depends}
dpkg-gencontrol: warning: Depends field of package libgphoto2-6-dbgsym: unknown substitution variable ${shlibs:Depends}
dh_gencontrol -plibgphoto2-6 -- -Vudev-hotplug='udev (>= 201)'
make[1]: Leaving directory '/home/ubuntu-xenial/pascal/Downloads/Launchpad/libgphoto2/libgphoto2-2.5.9'
   dh_md5sums -O--parallel
   dh_builddeb -O--parallel

It would be nice for Ubuntu to update the build in all supported releases.

The rebuild will be available in my PPA soon:
https://launchpad.net/~anton+/+archive/ubuntu/photo-video-apps

However beware that this PPA is yet NOT to be added on any system as it's still unstable and miss some dependencies i.e. if installing a package you will be faced with a partial upgrade which in turn will remove some of your already installed apps.