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-libssp.sh

@@ -0,0 +1,43 @@
+#!/bin/sh
+
+set -e
+
+if [ $# -lt 1 ]; then
+    echo $0 dest
+    exit 1
+fi
+PREFIX="$1"
+export PATH=$PREFIX/bin:$PATH
+
+: ${CORES:=4}
+: ${ARCHS:=${TOOLCHAIN_ARCHS-i686 x86_64 armv7 aarch64}}
+
+if [ ! -d libssp ]; then
+    svn checkout svn://gcc.gnu.org/svn/gcc/tags/gcc_7_3_0_release/libssp
+fi
+
+cp libssp-Makefile libssp/Makefile
+
+cd libssp
+
+# 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.
+cp config.h.in config.h
+for i in HAVE_FCNTL_H HAVE_INTTYPES_H HAVE_LIMITS_H HAVE_MALLOC_H \
+    HAVE_MEMMOVE HAVE_MEMORY_H HAVE_MEMPCPY HAVE_STDINT_H HAVE_STDIO_H \
+    HAVE_STDLIB_H HAVE_STRINGS_H HAVE_STRING_H HAVE_STRNCAT HAVE_STRNCPY \
+    HAVE_SYS_STAT_H HAVE_SYS_TYPES_H HAVE_UNISTD_H HAVE_USABLE_VSNPRINTF; do
+    cat config.h | sed 's/^#undef '$i'$/#define '$i' 1/' > tmp
+    mv tmp config.h
+done
+cat ssp/ssp.h.in | sed 's/@ssp_have_usable_vsnprintf@/define/' > ssp/ssp.h
+
+for arch in $ARCHS; do
+    mkdir -p build-$arch
+    cd build-$arch
+    make -f ../Makefile -j$CORES CROSS=$arch-w64-mingw32-
+    cp libssp.a $PREFIX/$arch-w64-mingw32/lib
+    cp libssp.a $PREFIX/$arch-w64-mingw32/lib/libssp_nonshared.a
+    cd ..
+done