fix C_FLAGS typo (#3538)

diff --git a/aten/CMakeLists.txt b/aten/CMakeLists.txt
index cbdfcde..4b4911a 100644
--- a/aten/CMakeLists.txt
+++ b/aten/CMakeLists.txt
@@ -27,7 +27,7 @@
 # C++11
 set(CMAKE_CXX_FLAGS "--std=c++11 ${CMAKE_CXX_FLAGS}")
 
-set(CMAKE_C_FLAGS "-fexceptions ${CMAKE_CXX_FLAGS}")
+set(CMAKE_C_FLAGS "-fexceptions ${CMAKE_C_FLAGS}")
 set(CMAKE_CXX_FLAGS "-fexceptions ${CMAKE_CXX_FLAGS}")