driver list: move more driver presence conditions from build rules to source files
Move conditional inclusion of source files in the build process out of
build rules (automake, and cmake). Unconditionally build the files
instead. While internal checks in the source code determine whether to
register a driver.
This simplifies build rules. And better reflects what's happening in the
sigrok project sources. Build rules really need not be aware of, even
less duplicate internal details of the source code. That's what attempts
to automatically create the driver list try so hard to avoid.
Internal conditions whether to register a driver could be arbitrarily
complex. The implementation of drivers here is simple, the project's
focus is on the correct and portable construction of the drivers list.