use strict CFLAGS to catch common mistakes in ci pipeline

A number of common coding errors are missed by the compiler. They turn up only after release once packages are build with full CFLAGS.

Adjust the gitlab CI to catch such errors:

-Wall enables a number of diagnostics

-Wno-deprecated-declarations will hide a bogus warning. As long as an API exists it can be used. Noone beside a curious developer will actually take the time to do research and look for the replacment. In the wild this warning is just noise in the build log.

-Werror=implicit-function-declaration will point out missing includes. In case such errors are not fixed, the resulting code will assume int for function arguments and function return values.

-Werror=return-type will point out incorrect return values. The caller will most likely receive just garbage.

Signed-off-by: Olaf Hering olaf@aepfle.de