[libc++abi] Do not define -Dcxxabi_shared_EXPORTS when building libc++abi

CMake sets adds that definition automatically, but we don't need
or use it.

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@373940 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index 8d4eb79..0bb164d 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -176,7 +176,9 @@
                           SOVERSION
                             "1"
                           VERSION
-                            "1.0")
+                            "1.0"
+                          DEFINE_SYMBOL
+                            "")
 
   if(LIBCXXABI_ENABLE_PIC)
     set_target_properties(cxxabi_shared PROPERTIES POSITION_INDEPENDENT_CODE ON)