make/[Makefile|*.mk): Improve variable caching

This commit aims at improving speed by using better caching.
- Makefile, cache.mk: Split .cache.mk up
    To allow caching of runtime path variables which are
    project-specific, split .cache.mk up in .cache-project.mk and
    .cache-projects.mk
- ldlibpath.mk: Cache ldlibpath, exepath and pythonpath
    Place the output of $(call proj_query ldlibpath), $(call
    proj_query, exepath) and $(call proj_query pythonpath) in
    JW_PKG_LD_LIBRARY_PATH, JW_PKG_EXE_PATH, and JW_PKG_PYTHON_PATH
    respectively, and cache the variables in make/.project-cache.mk.
- cache.mk: Use = instead of :=
    Recursively expanded variables are nearly as fast as := variables
    if the assigned value is a fixed string. And sometimes it's not,
    rightly so, because variables get assigned below, as with
    JW_PKG_XXX for instance.
- cache.mk: Use $(TOPDIR) as variable values
    Replace absolute references to project's topdir by $(TOPDIR) with
    sed. As soon as the project queries produce absolute paths, they
    will be transformed into relative paths which allow the code base
    to be moved to a different location and still remain functional
    without a rebuild.
Signed-off-by: Jan Lindemann <jan@janware.com>

make/Makefile

@@ -12,7 +12,8 @@ endif
 
 include $(JWBDIR)/make/dev-utils.mk
 
-all:
+all: cache-projects
+clean: clean-cache-projects
 
 ifeq ($(TARGET),mingw)
 install: $(INSTALL_MAKEDIR)/winres.rc.tmpl