build: don't add -fPIC on Windows

`-fPIC` doesn't make much sense for Windows, since Windows DLLs aren't compiled
position independent and are instead relocated at runtime.

Patch by Shoaib Meenai!

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@280413 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed