build: use the platform dependent library prefix/suffix

Use the cmake variables to get the platform dependent values for the
static library prefix and suffix, which can be different from the Unix
preference for "lib", ".a" (e.g. Windows uses "", ".lib" respectively).

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@290939 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CMakeLists.txt b/lib/CMakeLists.txt
index 857aaaa..41b9ced 100644
--- a/lib/CMakeLists.txt
+++ b/lib/CMakeLists.txt
@@ -232,7 +232,8 @@
     if (TARGET ${LIBCXX_CXX_ABI_LIBRARY})
       set(MERGE_ARCHIVES_ABI_TARGET "$<TARGET_LINKER_FILE:${LIBCXX_CXX_ABI_LIBRARY}>")
     else()
-      set(MERGE_ARCHIVES_ABI_TARGET "lib${LIBCXX_CXX_ABI_LIBRARY}.a")
+      set(MERGE_ARCHIVES_ABI_TARGET
+          "${CMAKE_STATIC_LIBRARY_PREFIX}${LIBCXX_CXX_ABI_LIBRARY}${CMAKE_STATIC_LIBRARY_SUFFIX}")
     endif()
     add_custom_command(TARGET cxx_static POST_BUILD
     COMMAND