Some of the smaller Debian buildds don't like running 500 threads at once.

--- a/test/concurrency.c
+++ b/test/concurrency.c
@@ -12,7 +12,7 @@
 #ifdef USE_VALGRIND
 #define THREAD_COUNT 50
 #else
-#define THREAD_COUNT 500
+#define THREAD_COUNT 50
 #endif
 
 void* test_runner(void*);
