Compile error: gsc_mysql.cpp:5:25: fatal error: mysql/mysql.h

[Ni3ls]

after apt-get update

Err https://packages.sury.org jessie/main amd64 Packages
  HttpError404
Get:91 https://packages.sury.org jessie/main i386 Packages
Err https://packages.sury.org jessie/main i386 Packages
  HttpError404
Fetched 26.2 MB in 4s (5,568 kB/s)
W: Failed to fetch https://packages.sury.org/php/dists/jessie/main/binary-amd64/Packages  HttpError404

W: Failed to fetch https://packages.sury.org/php/dists/jessie/main/binary-i386/Packages  HttpError404

E: Some index files failed to download. They have been ignored, or old ones used instead.

and after apt-get install libmysqlclient-dev:i386

Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 libmysqlclient-dev:i386 : Depends: zlib1g-dev:i386 but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

So i guess i have to use a more stable debian version?

[IzNoGoD]

The mirror you're using for your libmysqlclient is updated. It does only have buster and stretch distro packages listed now.

Also, you were using a jessie package from there, while jessie is not the version of debian that you're running.

Update your server (best through a full reinstall at this point)