dependencies: Allow pkg-config to define variables

The test used by the new define_variable parameter was using the
includedir directory. However, in order to get a successful test,
the includedir variables needs to be relative to the prefix
variable, otherwise the replacemente will not have any effect.

This changes uses the prefix variable itself because we can
assure that it will be present.

pkg-config enables to define variables by using the define-variable
option. This allows some packages to redefine relative paths, so
files can be installed in the same relative paths but under prefix.

Adds a test so that this is catched later.

Closes https://github.com/mesonbuild/meson/issues/1937

This class now consolidates a lot of the logic that each external
dependency was duplicating in its class definition.

All external dependencies now set:

* self.version
* self.compile_args and self.link_args
* self.is_found (if found)
* self.sources
* etc

And the abstract ExternalDependency class defines the methods that
will fetch those properties. Some classes still override that for
various reasons, but those should also be migrated to properties as
far as possible.

Next step is to consolidate and standardize the way in which we call
'configuration binaries' such as sdl2-config, llvm-config, pkg-config,
etc. Currently each class has to duplicate code involved with that
even though the format is very similar.

Currently only pkg-config supports multiple version requirements, and
some classes don't even properly check the version requirement. That
will also become easier now.

Includes tests for both of them.