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.

Dockerfile

@@ -2,7 +2,8 @@ FROM ubuntu:16.04
 
 RUN apt-get update -qq && apt-get install -qqy \
     git wget bzip2 file unzip libtool pkg-config cmake build-essential \
-    automake yasm gettext autopoint vim python git-svn ninja-build && \
+    automake yasm gettext autopoint vim python git-svn ninja-build \
+    subversion && \
     apt-get clean -y && \
     rm -rf /var/lib/apt/lists/*
 
@@ -18,7 +19,7 @@ ENV TOOLCHAIN_PREFIX=/opt/llvm-mingw
 ENV TOOLCHAIN_ARCHS="i686 x86_64 armv7 aarch64"
 
 # Build everything and clean up, in one step
-COPY *.sh ./
+COPY *.sh libssp-Makefile ./
 COPY wrappers/*.sh ./wrappers/
 RUN ./build-all.sh $TOOLCHAIN_PREFIX && \
     ./strip-llvm.sh $TOOLCHAIN_PREFIX && \