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
1 file changed