btl/usnic: amend Makefile.am fix from b4097626ab

Use $(AM_CPPFLAGS) in $(usnic_btl_run_tests_CPPFLAGS) so that we don't
have to replicate hard-coded values.

Signed-off-by: Jeff Squyres <jsquyres@cisco.com>
(cherry picked from commit 14563770a1d64c465ee1f205c9981de39970bb33)

opal/mca/btl/usnic/Makefile.am

@@ -104,9 +104,8 @@ libmca_btl_usnic_la_LDFLAGS = \
 libmca_btl_usnic_la_LIBADD = $(opal_ofi_LIBS)
 
 if OPAL_BTL_USNIC_BUILD_UNIT_TESTS
-usnic_btl_run_tests_CPPFLAGS = \
-    -DBTL_USNIC_RUN_TESTS_SYMBOL=\"opal_btl_usnic_run_tests\" \
-    -DOMPI_LIBMPI_NAME=\"$(OMPI_LIBMPI_NAME)\"
+usnic_btl_run_tests_CPPFLAGS = $(AM_CPPFLAGS) \
+    -DBTL_USNIC_RUN_TESTS_SYMBOL=\"opal_btl_usnic_run_tests\"
 usnic_btl_run_tests_SOURCES = test/usnic_btl_run_tests.c
 usnic_btl_run_tests_LDADD = -ldl
 bin_PROGRAMS = usnic_btl_run_tests