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/rpmdist.mk

@@ -140,7 +140,7 @@ pkg-update: pkgupdate.dist
 pkg-upload-local: pkg-upload-local.dist
 
 clean.rpm:
-	$(RM) -rf $(DIST_DIR) *.dist make/.cache.mk
+	$(RM) -rf $(DIST_DIR) *.dist make/.cache-project.mk make/.cache-projects.mk
 
 #ifeq ($(PROJECT_DESCR_FILE),)
 #