Fix CMake/MSVC when compiler-rt and llvm are built separately

Summary:
For some reason CMake can't find the `append` macro if LLVM is built separately and imported via `LLVM_CONFIG_PATH`.

Patch by Loo Rong Jie

Reviewers: rnk, vitalybuka

Reviewed By: rnk, vitalybuka

Subscribers: dberris, mgorny, llvm-commits, #sanitizers

Differential Revision: https://reviews.llvm.org/D43458

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@327876 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 4b953b2..a02ef95 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -339,7 +339,7 @@
   # it, but CMake doesn't seem to have a way to set linker flags for
   # individual static libraries, so we enable the suppression flag for
   # the whole compiler-rt project.
-  append("/IGNORE:4221" CMAKE_STATIC_LINKER_FLAGS)
+  set(CMAKE_STATIC_LINKER_FLAGS "${CMAKE_STATIC_LINKER_FLAGS} /IGNORE:4221")
 endif()
 
 add_subdirectory(include)