test: add a simple check, that -std=c99 working

test/pthread/Makefile.in

@@ -6,3 +6,5 @@ TESTS_DISABLED += cancellation-points
 EXTRA_LDFLAGS := -lpthread
 
 LDFLAGS_cancellation-points := -lrt
+
+CFLAGS_tst-c99 := -std=c99

test/pthread/tst-c99.c

@@ -0,0 +1,2 @@
+#include <pthread.h>
+int main(void) { return 0; }