[sanitizer] Max out allowed macOS deployment targets to 10.9

[kubamracek]

Summary

It's not a good idea to build the sanitizers with e.g. -DCMAKE_OSX_DEPLOYMENT_TARGET=10.12, because some deprecated functions that we intercept will cause build errors. Let's limit the allowed deployment targets to 10.9 (which is the default anyway), and warn when it's set above.

compiler-rt/trunk/cmake/config-ix.cmake

if(NOT SANITIZER_MIN_OSX_VERSION)
     if(MACOSX_VERSION_MIN_FLAG)
       set(SANITIZER_MIN_OSX_VERSION "${CMAKE_MATCH_1}")
     elseif(CMAKE_OSX_DEPLOYMENT_TARGET)
       set(SANITIZER_MIN_OSX_VERSION ${CMAKE_OSX_DEPLOYMENT_TARGET})
     else()
       set(SANITIZER_MIN_OSX_VERSION 10.9)
     endif()
     if(SANITIZER_MIN_OSX_VERSION VERSION_LESS "10.7")
-      message(FATAL_ERROR "Too old OS X version: ${SANITIZER_MIN_OSX_VERSION}")
+      message(FATAL_ERROR "macOS deployment target '${SANITIZER_MIN_OSX_VERSION}' is too old.")
+    endif()
+    if(SANITIZER_MIN_OSX_VERSION VERSION_GREATER "10.9")
+      message(WARNING "macOS deployment target '${SANITIZER_MIN_OSX_VERSION}' is too new, setting to '10.9' instead.")
+      set(SANITIZER_MIN_OSX_VERSION 10.9)
     endif()
   endif()
 
   # We're setting the flag manually for each target OS
   set(CMAKE_OSX_DEPLOYMENT_TARGET "")
   
   set(DARWIN_COMMON_CFLAGS -stdlib=libc++)
   set(DARWIN_COMMON_LINKFLAGS