Build libunwind with -Wno-dll-attribute-on-redeclaration

libunwind doesn't have the dllexport attributes on the original
declarations of functions, and clang normally warns about this.

build-libcxx.sh

@@ -109,7 +109,8 @@ build_all() {
             -DLIBUNWIND_ENABLE_STATIC=$STATIC \
             -DLIBUNWIND_ENABLE_CROSS_UNWINDING=FALSE \
             -DLIBUNWIND_STANDALONE_BUILD=TRUE \
-            -DCMAKE_CXX_FLAGS="-nostdinc++ -I$LIBCXX/include" \
+            -DCMAKE_CXX_FLAGS="-nostdinc++ -I$LIBCXX/include -Wno-dll-attribute-on-redeclaration" \
+            -DCMAKE_C_FLAGS="-Wno-dll-attribute-on-redeclaration" \
             -DCMAKE_SHARED_LINKER_FLAGS="-lpsapi" \
             ..
         make -j$CORES