Integrate libgphoto2/libgphoto2_port/configure.ac into libgphoto2/configure.ac

[ndim]

Is your feature request related to a problem? Please describe.
Running configure --help in a libgphoto2 source tree only shows the configure options for libgphoto2, but not for libgphoto2_port. That is confusing, and bad documentaiton.

Describe the solution you'd like
I would like to integrate libgphoto2/libgphoto2_port/configure.ac into libgphoto2/configure.ac so that a single configure --help run shows all configuration options. That also should accelerate the time needed to run autoreconf (by about a factor of 2) and configure for libgphoto2.

Describe alternatives you've considered
Keep explaining libgphoto2_port options to libgphoto2 users, and keep waiting longer for autoreconf and configure to finish.

Additional context
This is completely unrelated to whether the binaries built (libgphoto2.so and libgphoto2_port.so) are two separate ones or one integrated one.

This issue is so that I remember to do this, and so I can collect related comments here.

[ndim]

One interesting part will be whether the translations for libgphoto2 and libgphoto2_port can be kept separate as they are right now, even with a single common integrated configure.ac.

[jtojnar]

Alternative might be switching to something like Meson build system, which has native support for subprojects. That way you could maintain a clean separation of the libraries while able to pass options to libgphoto2_port.

[ndim]

JFTR, it is possible to keep the libgphoto2 po/ subdir separate from the libgphoto2_port po/ subdir even with a single configure.ac.