Move some build defines to a separate config.h file

[Vinícius dos Santos Oliveira]

Currently build exposes ALL configure-time defines to the sources appending arguments to the compilder command line.

This choice is suboptimal, because exposes unneeded defines to files not wanting it and, worse, it will recompile every source file if you change the configure-time defines.

Solution: Create a config.h (preferably out-of-tree in shadow builds) build-time generated file to aggregate all these changes. The config.h file could have its own directory, then the -I compiler arg wouldn't expose more files than needed.