Express variation in C pre-processor flags via CPPFLAGS.

* Using CFLAGS for this was incorrect and resulted in unnecessarily complex
  and divergent Makefile recipes.

tests/Makefile.am

@@ -51,8 +51,8 @@ pio_write_LDADD = ../src/libcdipio.la ../src/libcdi.la $(PPM_CORE_LIBS) $(YAXT_L
 pio_write_deco2d_LDADD = ../src/libcdipio.la ../src/libcdi.la $(PPM_CORE_LIBS) $(YAXT_LIBS) $(MPI_C_LIB)
 TESTS +=  test_resource_copy_mpi_run
 check_PROGRAMS += test_resource_copy_mpi
-test_resource_copy_mpi_LDADD = ../src/libcdipio.la ../src/libcdi.la
-test_resource_copy_mpi_CFLAGS = $(AM_CFLAGS) $(CFLAGS) \
+test_resource_copy_mpi_LDADD = ../src/libcdipio.la ../src/libcdi.la $(UUID_C_LIB)
+test_resource_copy_mpi_CPPFLAGS = $(AM_CPPFLAGS) $(CPPFLAGS) \
 	-DMPI_MARSHALLING
 else
 pio_write_LDADD = $(LDADD)