[PATCH mig] tests: preprocess .defs with target compiler, not host cpp

run_mig used bare cpp, so cross-test preprocessing saw host
macros, not the target's.  Switch to "$CC -E -x c" -- matches
mig.in's production preprocessing.
---
 tests/test_lib.sh | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tests/test_lib.sh b/tests/test_lib.sh
index 43ef738..7cbe639 100644
--- a/tests/test_lib.sh
+++ b/tests/test_lib.sh
@@ -38,7 +38,7 @@ run_mig () {
   file="$1"
   module="$2"
   echo "Generating stubs for $module..."
-  cpp $file -I$TEST_DIR | $MIGCOM -server $module-server.c -user $module-user.c -header $module-header.h
+  ${CC:-cc} -E -x c $file -I$TEST_DIR | $MIGCOM -server $module-server.c -user $module-user.c -header $module-header.h
 }
 
 test_module () {
-- 
2.54.0