message: Fix a small issue when building ex18/ex18p with GNU make and
  out-of-source.
diff: @@ -58,9 +58,9 @@ $(SUBDIRS_ALL) $(SUBDIRS_TEST) $(SUBDIRS_CLEAN):
  $(MAKE) -C $(@D) $(@F)
  # Additional dependencies
  -ex18: ex18.hpp
  +ex18: $(SRC)ex18.hpp
  ifeq ($(MFEM_USE_MPI),YES)
  -ex18p: ex18.hpp
  +ex18p: $(SRC)ex18.hpp
  endif
  MFEM_TESTS = EXAMPLES
  