Split out building widl to a separate shell script

When cross compiling a llvm-mingw toolchain, we need to build widl
for the intended host environment as well.

build-mingw-w64-widl.sh

@@ -0,0 +1,29 @@
+#!/bin/sh
+
+set -e
+
+if [ $# -lt 1 ]; then
+    echo $0 dest
+    exit 1
+fi
+PREFIX="$1"
+# Not adding $PREFIX/bin to $PATH here, since that is expected to be
+# a cross toolchain; the compiler is expected to be in $PATH already here.
+
+: ${CORES:=4}
+: ${ARCHS:=${TOOLCHAIN_ARCHS-i686 x86_64 armv7 aarch64}}
+
+if [ -n "$HOST" ]; then
+    CONFIGFLAGS="--host=$HOST"
+    CROSS_NAME=$HOST-
+fi
+
+cd mingw-w64/mingw-w64-tools/widl
+for arch in $ARCHS; do
+    mkdir -p build-$CROSS_NAME$arch
+    cd build-$CROSS_NAME$arch
+    ../configure --prefix=$PREFIX --target=$arch-w64-mingw32 $CONFIGFLAGS INSTALL="install -s"
+    make -j$CORES
+    make install
+    cd ..
+done