Keep downloaded files in a separate directory

As a side effect, this re-enables building the Windows installer on Linux hosts.

packaging/windows/buildpackage.sh

@@ -0,0 +1,25 @@
+#!/bin/bash
+
+TAG="$1"
+BUILTDIR="$2"
+SRCDIR="$3"
+OUTPUTDIR="$4"
+
+if [ -x /usr/bin/makensis ]; then
+    pushd "$SRCDIR" >/dev/null
+    popd >/dev/null
+    if [[ ! -f /usr/share/nsis/Include/nsProcess.nsh &&  ! -f /usr/share/nsis/Plugin/nsProcess.dll ]]; then
+        echo "Installing NsProcess NSIS plugin in /usr/share/nsis"
+        sudo unzip -qq -o ${SRCDIR}/thirdparty/download/NsProcess.zip 'Include/*' -d /usr/share/nsis
+        sudo unzip -qq -j -o ${SRCDIR}/thirdparty/download/NsProcess.zip 'Plugin/*' -d /usr/share/nsis/Plugins
+    fi
+    echo "Building Windows setup.exe"
+    makensis -V2 -DSRCDIR="$BUILTDIR" -DDEPSDIR="${SRCDIR}/thirdparty/download/windows" OpenRA.nsi
+    if [ $? -eq 0 ]; then
+        mv OpenRA.Setup.exe "$OUTPUTDIR"/OpenRA-$TAG.exe
+    else
+        exit 1
+    fi
+else
+    echo "Skipping Windows setup.exe build due to missing NSIS"
+fi