Currently PkgConfig takes language as a keyword parameter in position 3,
while the others take it as positional in position 2. Because most
dependencies don't actually set a language (they use C style linking),
using a positional argument makes more sense. ExtraFrameworkDependencies
is even more different, and duplicates some arguments from the base
ExternalDependency class.
For later changes I'm planning to make having all of the dependencies
use the same signature is really, really helpful.
Some slight refactoring for the dependency classes and
I switched the elbrus compiler to the GnuLikeCompiler.
This is also the correct use according to the documentation
of GnuLikeCompiler.