Fix the CMake build on Mac when setting MACOSX_DEPLOYMENT_TARGET=10.6

The build file was trying to use LIBCXXABI_VERSION, which isn't set
anywhere.

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@215465 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index a3fdf6c..774ef27 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -66,7 +66,7 @@
     list(APPEND compile_flags "-U__STRICT_ANSI__")
     list(APPEND link_flags
       "-compatibility_version 1"
-      "-current_version ${LIBCXXABI_VERSION}"
+      "-current_version 1"
       "-install_name /usr/lib/libc++abi.1.dylib"
       "/usr/lib/libSystem.B.dylib")
   else()