Make sure Android builds are redumped upon changes to shortlisp

Make sure Android builds are redumped upon changes to shortlisp

* build-aux/makecounter.sh: New script.
* src/Makefile.in (abs_top_builddir): New variable.
(BUILD_COUNTER_OBJ): Define to build-counter.o
if compiling for Android.
(build-counter.c): New target.  Generate this file using
makecounter.sh upon changes to lisp.mk or shortlisp.
(lisp.mk): Make and load relative to abs_top_builddir.
(emacs$(EXEEXT)): Adjust acordingly.
(mostlyclean): Remove build-counter.c.