mk: add short target to build with debug options

[rjarry]

Add a debug target that will force BUILDTYPE=debug and SANITIZE=address.

[david-marchand]

One problem with this patch is that the debug target does not "configure" the sources.

Example:

$ git clean -xffd
$ make
...
Found ninja-1.11.1 at /usr/bin/ninja
ninja: Entering directory `build'
[704/704] Linking target grout

$ make debug
ninja: Entering directory `build'
ninja: no work to do.

[rjarry]

For this we would have to use meson configure and it can be a bit fragile. Sometimes, not all sources get recompiled with new options.

rm -rf build && make debug

Seems safer but I don't want to always delete everything.