Turn on clang -fcolor-diagnostics.

I'm not sure why these aren't on by default, but Chromium does this too.
Colors are nice.

Change-Id: I7d7bf006014e9f40ec2f48290ad8fe7a70c1cfce
Reviewed-on: https://boringssl-review.googlesource.com/18704
Reviewed-by: Steven Valdez <svaldez@google.com>
Commit-Queue: David Benjamin <davidben@google.com>

CMakeLists.txt

@@ -39,7 +39,7 @@ endif()
 if(CMAKE_COMPILER_IS_GNUCXX OR CMAKE_CXX_COMPILER_ID MATCHES "Clang")
   set(C_CXX_FLAGS "-Wall -Werror -Wformat=2 -Wsign-compare -Wmissing-field-initializers -Wwrite-strings -ggdb -fvisibility=hidden -fno-common")
   if(CMAKE_CXX_COMPILER_ID MATCHES "Clang")
-    set(C_CXX_FLAGS "${C_CXX_FLAGS} -Wnewline-eof")
+    set(C_CXX_FLAGS "${C_CXX_FLAGS} -Wnewline-eof -fcolor-diagnostics")
   else()
     # GCC (at least 4.8.4) has a bug where it'll find unreachable free() calls
     # and declare that the code is trying to free a stack pointer.