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/Makefile

@@ -5,22 +5,33 @@
 
 TOPDIR = ..
 
+CACHED_FILES      := defs.mk projects.mk platform.mk Makefile
+#CACHED_VARS        = $(shell sed '/^[ \t]*\([A-Z_]\+\) *\(.=\).*$$.shell/!d; s/ *.=.*//' $(CACHED_FILES))
+CACHED_VARS       := WHICH BROWSER SED RM PWD ID CUT TR AWK GETENT XARGS FIND PRINTF OS_TUPLE OS_NAME_VERSION OS_NAME CAT BIN_INSTALL SUDO
+#CACHED_VARS       += TARGET_ARCH RPM_ARCH
+CACHED_VARS       += TARGET_ARCH
+#CACHED_VARS       += DEVELOPMENT
+
 include $(TOPDIR)/make/proj.mk
 include $(MODDIR)/make/make.mk
 ifneq ($(MAKECMDGOALS),install-links)
-ifneq ($(MAKECMDGOALS),do-install-links)
-include $(MODDIR)/make/dirs.mk
-endif
+  ifneq ($(MAKECMDGOALS),do-install-links)
+    include $(MODDIR)/make/dirs.mk
+  endif
 endif
 
 include $(MODDIR)/make/dev-utils.mk
+#include $(MODDIR)/make/cache.mk
+
+all:
 
 ifeq ($(TARGET),mingw)
 install: $(INSTALL_MAKEDIR)/winres.rc.tmpl
 endif
 
+grep-vars:
+	@sed '/^ *[A-Z]/!d; s/^[        ]*//g; s/[=+?:].*//g; s/[       ]*//g' *.mk | sort -u
+
 $(INSTALL_MAKEDIR)/%.tmpl: %.tmpl $(TOPDIR)/dir_install_MAKE.done
 	$(INSTALL) -o $(MAKEOWNER) -g $(MAKEGROUP) -m $(MAKEMODE) $< $@A
 
-grep-vars:
-	@sed '/^ *[A-Z]/!d; s/^[        ]*//g; s/[=+?:].*//g; s/[       ]*//g' *.mk | sort -u