Fix build nondeterminism by deleting archives before recreating

Otherwise, `ar rcs` will add to the previous archive, creating the
possibility of multiple functions with the same name in the archive.
The linker might not choose the most recent version, resulting in
undefined behavior.

Makefile.in

@@ -242,6 +242,7 @@ $(2)_lib_libarg     := $$(patsubst %, -l%, $$($(2)_lib_libs))
 $(2)_lib_libnames_shared	:= $$(if $$($(2)_install_shared_lib),lib$(1).so,)
 
 lib$(1).a : $$($(2)_objs) $$($(2)_c_objs)
+	rm -f $$@
 	$(AR) rcs $$@ $$^
 lib$(1).so : $$($(2)_objs) $$($(2)_c_objs) $$($(2)_lib_libnames_shared) $$($(2)_lib_libnames)
 	$(LINK) -shared -o $$@ $(if $(filter Darwin,$(shell uname -s)),-install_name $(install_libs_dir)/$$@) $$^ $$($(2)_lib_libnames) $(LIBS)