Build unwind-seh.c from libgcc instead of libunwind, if SEH is in use

build-libcxx.sh

@@ -24,6 +24,9 @@ if [ ! -d libcxx ]; then
     git clone -b master https://github.com/llvm-mirror/libcxx.git
     CHECKOUT_LIBCXX=1
 fi
+if [ ! -d libgcc ]; then
+    svn checkout svn://gcc.gnu.org/svn/gcc/tags/gcc_7_3_0_release/libgcc
+fi
 if [ -n "$SYNC" ] || [ -n "$CHECKOUT_LIBUNWIND" ]; then
     cd libunwind
     [ -z "$SYNC" ] || git fetch
@@ -48,6 +51,24 @@ MERGE_ARCHIVES=$(pwd)/merge-archives.sh
 
 cd libunwind
 for arch in $ARCHS; do
+    if $arch-w64-mingw32-clang -E -dM - < /dev/null 2>/dev/null | grep -q __SEH__; then
+        cd ../libgcc
+        mkdir -p build-$arch
+        cd build-$arch
+        rm -f *.h
+        cp ../unwind-seh.c .
+        cp ../unwind-generic.h unwind.h
+        touch tconfig.h
+        touch tsystem.h
+        touch coretypes.h
+        touch tm.h
+        echo "#define ATTRIBUTE_UNUSED __attribute__ ((__unused__))" >> tconfig.h
+        $arch-w64-mingw32-clang -O2 -Wall -c unwind-seh.c -o unwind-seh.o
+        llvm-ar rcs libunwind.a unwind-seh.o
+        cp libunwind.a $PREFIX/$arch-w64-mingw32/lib
+        cd ../../libunwind
+        continue
+    fi
     mkdir -p build-$arch
     cd build-$arch
     # If llvm-config and the llvm cmake files are available, -w gets added