Fix cmake debian dependencies.

[TallFurryMan]

Additional fix to #975 to allow cmake to generate a debian package for Ubuntu 20.04.

[agalasso]

@pchev do you want to review this before I merge it?

[pchev]

I not use the packaging cmake, but for me the change look good.

[TallFurryMan]

Tested locally here, I need to report I had an issue with LD_LIBRARY_PATH on libtoupcam.so, which resided in /usr/local/lib and was not readily available in the observatory. I have not investigated why this specific library, and not others, was causing an issue. There might be a rpath issue or something, or just my own system config specifics.

[TallFurryMan]

OK this is simply because the lib is in a subdirectory. Running ldd against phd2.bin returns (subset) :

... 
libASICamera2.so => /usr/local/lib/libASICamera2.so (0x00007fbcafcb4000)
libtoupcam.so => not found                           
libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007fbcafb65000)
... 

And:

$ find /usr/local -name libtoupcam.so
/usr/local/lib/phd2/libtoupcam.so

So that lib doesn't seem to be stored at the right place when packaging with cmake. I can probably provide a PR later today.

[pchev]

The missing library path is only when you open phd2.bin directly. The script phd2.sh is installed as /usr/bin/phd2 to solve that.