PROTON-2225: compile c-threaderciser with COMPILE_LANGUAGE_FLAGS (#250)

diff --git a/c/tests/CMakeLists.txt b/c/tests/CMakeLists.txt
index 647e486..86a4a41 100644
--- a/c/tests/CMakeLists.txt
+++ b/c/tests/CMakeLists.txt
@@ -88,7 +88,8 @@
     option(THREADERCISER "Run the threaderciser concurrency tests" ${DEFAULT_THREADERCISER})
     if (THREADERCISER)
       add_executable(c-threaderciser threaderciser.c)
-      set_target_properties(c-threaderciser PROPERTIES COMPILE_FLAGS "${CMAKE_C_FLAGS} ${C_WARNING_FLAGS}")
+      set_target_properties(c-threaderciser
+        PROPERTIES COMPILE_FLAGS "${COMPILE_LANGUAGE_FLAGS} ${CMAKE_C_FLAGS} ${C_WARNING_FLAGS}")
       pn_add_test(
         EXECUTABLE
         NAME c-threaderciser