build: add proper dependency checking to configure.ac

build: add proper dependency checking to configure.ac

We build several tools inside of this repository and we need to make sure
all dependencies are met. Furthermore, we must make sure that default
values are recognized even on dependencies.
Hence, this reworks the whole configure.ac handling and does some renaming
of the constants that are used all over the source.

We need to test this some more weeks, but it should finally be a proper
autotools handling that we can release with kmscon-6.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>