make: Further improve build time

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>

make/cache.mk

@@ -0,0 +1,22 @@
+# ----- define these variables
+# CACHED_FILES     = $(TOPDIR)/VERSION
+# CACHED_VARS      = VERSION
+
+CACHE_FILE_MK ?= .cache.mk
+
+# if not checked against MAKECMDGOALS, a standard rule kicks in from rules.mk,
+# and $(CACHE_FILE_MK) is remade with the clean target
+ifeq ($(findstring clean,$(MAKECMDGOALS)),)
+all: $(CACHE_FILE_MK)
+$(CACHE_FILE_MK): $(CACHED_FILES) $(MODDIR)/make/cache.mk
+ifeq ($(wildcard $(CACHE_FILE_MK)),)
+	@echo $(foreach v,$(CACHED_VARS),$v := $(value $(v))EOL) | $(SED) 's/EOL */\n/g;' | $(GREP) . | tee $@.tmp
+	mv $@.tmp $@
+else
+	rm $@
+	make $@
+endif
+endif
+clean: clean-cache
+clean-cache:
+	$(RM) -f $(CACHE_FILE_MK)