Build libssp from gcc

gcc/libssp's configure script runs checks for flags that clang doesn't
implement. We actually just need to set a few HAVE defines and compile
the .c sources, so just check out this subdirectory using svn and build
it using a custom makefile.

build-all.sh

@@ -13,3 +13,4 @@ PREFIX="$1"
 ./build-mingw-w64.sh $PREFIX
 ./build-compiler-rt.sh $PREFIX
 ./build-libcxx.sh $PREFIX
+./build-libssp.sh $PREFIX