Make missing prototypes warning work in clang/C++.

in GCC, -Wmissing-prototypes is only in C and the C++ spelling is
-Wmissing-declarations.
https://gcc.gnu.org/onlinedocs/gcc-7.1.0/gcc/Warning-Options.html#Warning-Options

In clang, -Wmissing-prototypes is in both C/C++ and
-Wmissing-declarations means you forgot to name a typedef or something?
https://clang.llvm.org/docs/DiagnosticsReference.html#wmissing-prototypes
https://clang.llvm.org/docs/DiagnosticsReference.html#wmissing-declarations

Pass clang slightly different flags to account for this mess.

Change-Id: I574ff7ecf32ba5908355639de83e8a102dab5f04
Reviewed-on: https://boringssl-review.googlesource.com/17784
Reviewed-by: Steven Valdez <svaldez@google.com>

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index deab75b..a71ebf7 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -43,6 +43,16 @@
   endif()
   set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${C_CXX_FLAGS} -Wmissing-prototypes -Wold-style-definition -Wstrict-prototypes")
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11 ${C_CXX_FLAGS} -Wmissing-declarations")
+  # In GCC, -Wmissing-declarations is the C++ spelling of -Wmissing-prototypes
+  # and using the wrong one is an error. In Clang, -Wmissing-prototypes is the
+  # spelling for both and -Wmissing-declarations is some other warning.
+  #
+  # https://gcc.gnu.org/onlinedocs/gcc-7.1.0/gcc/Warning-Options.html#Warning-Options
+  # https://clang.llvm.org/docs/DiagnosticsReference.html#wmissing-prototypes
+  # https://clang.llvm.org/docs/DiagnosticsReference.html#wmissing-declarations
+  if(CMAKE_CXX_COMPILER_ID MATCHES "Clang")
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wmissing-prototypes")
+  endif()
 elseif(MSVC)
   set(MSVC_DISABLED_WARNINGS_LIST
       "C4061" # enumerator 'identifier' in switch of enum 'enumeration' is not