build: use more portable spelling for flag

Use `CMAKE_LIBRARY_PATH_FLAG` instead of hard-coding it to -L.  This
silences a warning with cl which expects `/LIBPATH` instead.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@290938 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CMakeLists.txt b/lib/CMakeLists.txt
index 4383c07..857aaaa 100644
--- a/lib/CMakeLists.txt
+++ b/lib/CMakeLists.txt
@@ -28,8 +28,9 @@
   set(exclude_from_all EXCLUDE_FROM_ALL)
 endif()
 
-#if LIBCXX_CXX_ABI_LIBRARY_PATH is defined we want to add it to the search path.
-add_link_flags_if(LIBCXX_CXX_ABI_LIBRARY_PATH "-L${LIBCXX_CXX_ABI_LIBRARY_PATH}")
+# If LIBCXX_CXX_ABI_LIBRARY_PATH is defined we want to add it to the search path.
+add_link_flags_if(LIBCXX_CXX_ABI_LIBRARY_PATH
+                  "${CMAKE_LIBRARY_PATH_FLAG}${LIBCXX_CXX_ABI_LIBRARY_PATH}")
 
 add_library_flags_if(LIBCXX_COVERAGE_LIBRARY "${LIBCXX_COVERAGE_LIBRARY}")