[ccache] dependency information not updated correctly

[bruce robertson]

The context of my problem is building Android.

I have two trees I do builds in. Part of the compile line has an "-MD -MF
out/host/linux-x86/obj/EXECUTABLES/grxmlcompile_intermediates/gr_iface.d",
for instance, to create the dependency file for the compilation.

Quoting from the ccache-3.7 MANUAL.txt,
How ccache works
----------------
...                                            On a cache hit, ccache is able
to supply all of the correct compiler outputs (including all warnings,
dependency file, etc) from the cache.
^^^^^^^^^^^^^^^

So say initially the ccache is clean (easily caused by ccache -C). I
build totally in tree number 1. Now many dependency files will have full
pathnames of the compilers stddef.h, float.h, stdarg.h etc. files. E.g.,
 /ssd/repos/jb-ia-pc_std/prebuilts/gcc/linux-x86/x86/i686-linux-android-4.6/bin/../lib/gcc/i686-linux-android/4.6.x-google/include/stdarg.h

Perhaps there's a way to make these not be a full pathname?

Now if I go to tree number 2, re-source envsetup.sh, lunch <mytgt>, do
another full make. ccache will supply objects and dependency files to
help the process. Good. Everything is fine. Sort of. The dependency
files fetched from ccache refer to a tree 1's compiler's system files
rather than the tree I'm using.

Now I decide I'm done with tree number 1, I delete or just rename it. If
I now do a make in tree number 2, all objects that refer to the
"missing" compiler include files will be rebuilt as I suppose ccache
assumes I have done something consistent like renaming some underlying
dependency. <SPECULATION> But as all the files actually when compiled or
preprocessed have the same signatures there is now a cache hit and the
cache provides the dependency file and object. And the dependency file
has not been changed </SPECULATION>, so we will do the same thing next
time.

It's distracting to see all these "builds" happening when I haven't
changed my tree at all. And it takes a couple more minutes.

Bruce