Introduce a cache of the configuration

You may have noticed that running make is a little slow. Every time
that it is loaded, it will try to detect everything again. So,
instead, save the output and reload it the next time.

This is implemented by adding a rule that (re-)creates the
config.cache file, which is included by make. If the file doesn't
exist yet, make will first run the rule which creates it, then reload
itself.

You can also cause it to reconfigure by running "make configure".
Cache the Qt tools detected (and detect RCC)

Since we're caching the include flags and linker flags, we should also
cache the exact helper binaries we're running. This avoids getting
errors because the environment changed.
Add a HEADERS variable to the Makefile

Similar to the qmake variable of the same name, this lists (at least)
the headers that may need moc to be run on. Adding more headers is not
a problem.
Add a SOURCES variable to the Makefile, replacing OBJS

Instead of listing objects, let's list sources. This matches also what
qmake and most other buildsystems do. The notable exception is the
kernel.

The reason that listing the sources will be interesting is because I'm
about to add rules to create the dependency files.
Get the list of dependency includes from the SOURCES list
Generate the C++ dependencies at configure-time

We use the -MG preprocessor option to the compiler to ask it to check
all #includes and tell us what's missing. Then our own rules will
generate the moc and uic files that the .cpp #include.

Unfortunately, our rules make uic generate output in qt-ui/ for
qt-ui/*.ui, while the compiler generates rules for no directory. We
need to fake it by forcing the generation.
Make the "silent mode" compilation be optional only.

If you run
   make V=1

Then we'll output the full command-line. It's useful for debugging
problems with the build.
Prettify the msgfmt and linking arguments