Makefile: add a missing target libharvid dependency of src

Without the change parallel build fails sometimes by linking
too early on non-existent file:

    make -C src
    make[1]: Entering directory '/build/harvid/src'
    ld -r -b binary -o seek.o ../doc/seek.js
    ld -r -b binary -o logo.o ../doc/harvid.jpg
    make[1]: *** No rule to make target '../libharvid/libharvid.a', needed by 'harvid'.  Stop.

It's nest reproducible in `make --shuffle` mode:
  https://savannah.gnu.org/bugs/index.php?62100

The change makes sure 'libharvid' is built before 'src'.

Makefile

@@ -3,12 +3,15 @@ SUBDIRS = libharvid src doc
 
 default: all
 
-$(SUBDIRS)::
+$(SUBDIRS):
 	$(MAKE) -C $@ $(MAKECMDGOALS)
 
 all clean man install uninstall install-bin install-man uninstall-bin uninstall-man install-lib uninstall-lib: $(SUBDIRS)
 
+# final binary links against a library
+src: libharvid
+
 dist:
 	git archive --format=tar --prefix=harvid-$(VERSION)/ HEAD | gzip -9 > harvid-$(VERSION).tar.gz
 
-.PHONY: clean all subdirs install uninstall dist install-bin install-man uninstall-bin uninstall-man install-lib uninstall-lib
+.PHONY: clean all subdirs install uninstall dist install-bin install-man uninstall-bin uninstall-man install-lib uninstall-lib $(SUBDIRS)