fix c build flag (#2444)

Author: xuhancn

cmake/cpu/BuildFlags.cmake

@@ -35,7 +35,7 @@ if(MSVC)
 endif(MSVC)
 
 if(NOT MSVC)
-  set(CMAKE_C_FLAGS "${CMAKE_CXX_FLAGS} -fPIC")
+  set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -fPIC")
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fPIC")
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-narrowing")
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-missing-field-initializers")