Enable stronger format string checking |-Wformat=2|.

Also, factor out flags based to both the C and C++ compiler into a
single variable.

Change-Id: I432de0cc516e95a0d48146fae2dda8b7b3b38d4b
Reviewed-on: https://boringssl-review.googlesource.com/6922
Reviewed-by: David Benjamin <davidben@google.com>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index e07be54..19ee1ca 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -17,8 +17,9 @@
 endif()
 
 if(CMAKE_COMPILER_IS_GNUCXX OR CMAKE_CXX_COMPILER_ID MATCHES "Clang")
-  set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wall -Werror -Wsign-compare -Wmissing-field-initializers -ggdb -fvisibility=hidden")
-  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Werror -Wsign-compare -Wmissing-field-initializers -ggdb -std=c++0x -fvisibility=hidden")
+  set(C_CXX_FLAGS "-Wall -Werror -Wformat=2 -Wsign-compare -Wmissing-field-initializers -ggdb -fvisibility=hidden")
+  set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${C_CXX_FLAGS}")
+  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++0x ${C_CXX_FLAGS}")
 elseif(MSVC)
   set(MSVC_DISABLED_WARNINGS_LIST
       "C4100" # 'exarg' : unreferenced formal parameter