ebuild: follow symbolic links in find_newer_filtered_fn

Otherwise, changes in source code symbolically-linked into the
workspace will not be considered when deciding whether to invoke
e.g. Ninja.

This change is strongly preferable to asking people to remove
.mu_build_build_timestamp to force a Ninja run.

Deals reasonably gracefully with symbolic link loops, at least in
trivial cases: "find: File system loop detected"

Type: improvement

Signed-off-by: Dave Barach <dave@barachs.net>
Change-Id: I6666c84861de5e8b8aebcb94158f53526aded089

build-root/Makefile

@@ -416,7 +416,7 @@ find_filter += -and -not -path '*/.mu_build_*'
 find_newer_filtered_fn =			\
   (! -f $(1)					\
     || -n $(call find_newer_files_fn,$(1),$(3))	\
-    || -n "`find -H $(2)			\
+    || -n "`find -L $(2) 			\
 	      -type f				\
               -and -newer $(1)			\
 	      -and \( $(4) \)			\