Include config.h in all source files and in no headers

Include config.h in all source files and in no headers

This way, #include "config.h" is consistently the very
first thing the compiler sees when compiling any file.

Some source files currently don't use anything defined in config.h,
but it's easier and less error-prone to include it now to anticipate
possible future changes in those files, config.h or other headers.