build: fix erroneous overwriting of flags

Always use list(APPEND) as it will perform the desired action even if the list
is empty or previously unset.  The first set is harmless, however, the
subsequent set was overwriting the previous flag setup resulting in an improper
compilation command being generated.  This manifested as a build failure on
Linux when using cmake + ninja.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@203638 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed