Enable -Wstring-concatenation and silence warning.

Newer versions of Clang have a warning to detect "suspicious" uses of
string concatenation, where they think a comma or so was missing. It
flags a false positive in x509_test.cc, which we can silence with
parentheses. Fuchsia builds with this warning enabled, so enable it to
catch future instances.

I couldn't find official documentation on when this was added, but
empirically it's in my clang-12 but not my clang-11. That's recent
enough that adding a version check seems prudent. Unfortunately,
version-detecting Clang is complex because AppleClang uses completely
different versions. There's a handy table on Wikipedia that maps them.

Change-Id: I503c21d39bb5c68dda9bda6da693c7208f3af561
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/54785
Reviewed-by: Adam Langley <agl@google.com>
Auto-Submit: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 0864958..69169c4 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -160,6 +160,16 @@
     set(C_CXX_FLAGS "${C_CXX_FLAGS} -Wno-free-nonheap-object")
   endif()
 
+  # -Wstring-concatenation was added in Clang 12.0.0, which corresponds to
+  # AppleClang 13.0.0 per the table in
+  # https://en.wikipedia.org/wiki/Xcode#Toolchain_versions
+  if((CMAKE_C_COMPILER_ID STREQUAL "Clang" AND
+      CMAKE_C_COMPILER_VERSION VERSION_GREATER_EQUAL "12.0.0") OR
+     (CMAKE_C_COMPILER_ID STREQUAL "AppleClang" AND
+      CMAKE_C_COMPILER_VERSION VERSION_GREATER_EQUAL "13.0.0"))
+    set(C_CXX_FLAGS "${C_CXX_FLAGS} -Wstring-concatenation")
+  endif()
+
   if(CLANG OR CMAKE_C_COMPILER_VERSION VERSION_GREATER_EQUAL "7.0.0")
     set(C_CXX_FLAGS "${C_CXX_FLAGS} -Wimplicit-fallthrough")
   endif()