ocaml: Link mlguestfs.cma and .cmxa with local libguestfs

Commit e11583a03a ("ocaml: Don't embed -L../lib/.libs in final
mlguestfs.cma or .cmxa") was correct in itself, but as a side effect
caused mlguestfs.cma to be linked to the system libguestfs.  If system
libguestfs was not installed, the build would fail entirely with:

  ocamlmklib -o mlguestfs \
    -ldopt '-Wl,-z,relro -Wl,--as-needed  -Wl,-z,pack-relative-relocs -Wl,-z,now -specs=/usr/lib/rpm/redhat/redhat-hardened-ld -specs=/usr/lib/rpm/redhat/redhat-annobin-cc1  -Wl,--build-id=sha1  ' \
    libguestfsocaml_a-guestfs-c.o libguestfsocaml_a-guestfs-c-actions.o libguestfsocaml_a-guestfs-c-errnos.o ../common/utils/libguestfsocaml_a-cleanups.o ../common/utils/libguestfsocaml_a-stringlists-utils.o guestfs.cmo \
     \
    -lguestfs
  /usr/bin/ld: cannot find -lguestfs: No such file or directory

Fix (from Mohamed Akram) is to add -L../lib/.libs to the -ldopt flag.
This does not embed the option in the final mlguestfs.cma file, but
seems to only use it while building the library.

Updates: commit e11583a03a
Reported-by: Mohamed Akram
Fixes: https://github.com/libguestfs/libguestfs/issues/191

ocaml/Makefile.am

@@ -64,13 +64,13 @@ endif
 
 stamp-mlguestfs: libguestfsocaml.a $(guestfs_cmm)
 	$(OCAMLMKLIB) -o mlguestfs \
-	  -ldopt '$(LDFLAGS)' \
+	  -ldopt '$(LDFLAGS) -L../lib/.libs' \
 	  $(libguestfsocaml_a_OBJECTS) guestfs.cmo \
 	  $(LTLIBINTL) \
 	  -lguestfs
 if HAVE_OCAMLOPT
 	$(OCAMLMKLIB) -o mlguestfs \
-	  -ldopt '$(LDFLAGS)' \
+	  -ldopt '$(LDFLAGS) -L../lib/.libs' \
 	  $(libguestfsocaml_a_OBJECTS) guestfs.cmx \
 	  $(LTLIBINTL) \
 	  -lguestfs