Fix the CMake build when setting MACOSX_DEPLOYMENT_TARGET

If we don't reset CMAKE_OSX_DEPLOYMENT_TARGET, it will end up in
CFLAGS together with the -mmacosx-version-min that we set,
and the compiler errors about the conflict.

llvm-svn: 215468
diff --git a/compiler-rt/CMakeLists.txt b/compiler-rt/CMakeLists.txt
index a8f8cc3..f0d53ff 100644
--- a/compiler-rt/CMakeLists.txt
+++ b/compiler-rt/CMakeLists.txt
@@ -294,6 +294,7 @@
   else()
     set(SANITIZER_MIN_OSX_VERSION 10.6)
   endif()
+  set(CMAKE_OSX_DEPLOYMENT_TARGET "") # We're setting the flag manually below.
   set(DARWIN_osx_CFLAGS -mmacosx-version-min=${SANITIZER_MIN_OSX_VERSION})
   set(DARWIN_iossim_CFLAGS 
     -mios-simulator-version-min=7.0 -isysroot ${IOSSIM_SDK_DIR})