Build modernize: Object targets / Client and server build should share objects

[sfan5]

Currently building both minetest and minetestserver essentially builds the entirety of our source code twice.

But out of our 300+ source files only 43 contain a preprocessor macro that checks SERVER, which means the rest (in theory) don't differ between client and server builds.
A lot of these are headers which infects other files whenever included but this can be improved once the first step is made.

Step 1:
As it is now absence of SERVER indicates a client build. This becomes a risk with files that are built only once because of easy confusion.
These should be replaced with a macro CLIENT_BUILD which is defined to 0 or 1.
Then, if someone uses #if CLIENT_BUILD in a file that is supposed to be non-specific a clear compiler warning is raised and the build probably fails at some point anyway.

Step 2:
Common files are moved to an object target that is then linked into both the server and the client.