This commit sees several improvements to the build performance:
- Introduce cache.mk, which creates makefiles caching often used
variables, per tree and per project.
- Define more variables with := enclosed in condistions, instead of
defining them with ?=, because the RHS of ?= is expanded deferredly.
- Add more definitions for executables.
- Move some more specialized definitions out into specialized makefiles,
notably htdocs.mk and tmpl.mk
Signed-off-by: Jan Lindemann <jan@janware.com>
