[Libguestfs] [PATCH libguestfs] build: don't use automake-internal variable

[Jim Meyering]

Additional info, here's an excerpt from Makefile.in regenerated
after applying that change:

am__libguestfsocaml_a_SOURCES_DIST = guestfs_c.c guestfs_c_actions.c
@HAVE_OCAML_TRUE at am_libguestfsocaml_a_OBJECTS =  \
@HAVE_OCAML_TRUE@	libguestfsocaml_a-guestfs_c.$(OBJEXT) \
@HAVE_OCAML_TRUE@	libguestfsocaml_a-guestfs_c_actions.$(OBJEXT)
libguestfsocaml_a_OBJECTS = $(am_libguestfsocaml_a_OBJECTS)