add cross-compile recipe for msdos

- If you want to obtain the djgpp cross-compiler and tools/libs for MSDOS,
which is available for linux and macOS, you can use the following script
to obtain it:

       sh sys/msdos/fetch-cross-compiler.sh

That script won't install anything, it is just file fetches. It will
store the cross-compiler in subfolders of lib and the hints files are
configured to find it appropriately there.

Note: Both the fetch and the msdos cross-compile package target require
unzip and zip to be available on your host build system.

Cross-compiler bits:
       https://github.com/andrewwutw/build-djgpp
   and the pre-built binary for your platform from:
       https://github.com/andrewwutw/build-djgpp/releases/download/v3.0/
   and a DOS-extender (for including in msdos packaging) from
       http://sandmann.dotster.com/cwsdpmi/csdpmi7b.zip
   and pdcurses from:
       https://github.com/wmcbrine/PDCurses.git

The MSDOS cross-compile can then be carried out by specifying
CROSS_TO_MSDOS=1 on the make command line.

For example:
       make CROSS_TO_MSDOS=1 all
       make CROSS_TO_MSDOS=1 package

You can explicitly include tty and curses support if desired, otherwise
you'll end up with a tty-only cross-compile build:
       make WANT_WIN_TTY=1 WANT_WIN_CURSES=1 CROSS_TO_MSDOS=1 all

Also note that building the msdos targets using the make command
above, does not preclude you from building local linux or macOS
targets as well. Just drop the CROSS_TO_MSDOS=1 from the make
command line.

The cross-compiler hints additions are enclosed inside ifdef sections
and won't interfere with the non-cross-compile build in that case.

sys/msdos/fetch-cross-compiler.sh

@@ -0,0 +1,97 @@
+#!/bin/sh
+#set -x
+
+if [ -z "$TRAVIS_BUILD_DIR" ]; then
+	export DJGPP_TOP=$(pwd)/lib/djgpp
+else
+	export DJGPP_TOP="$TRAVIS_BUILD_DIR/lib/djgpp"
+fi
+
+if [ -z "$GCCVER" ]; then
+	export GCCVER=gcc1010
+fi
+
+if [ -z "$LUA_VERSION" ]; then
+	export LUA_VERSION=5.4.0
+fi
+
+if [ ! -d "$(pwd)/lib" ]; then
+	echo "Set up for Unix build and 'make fetch-lua' first."
+	exit 1
+fi
+
+#DJGPP_URL="https://github.com/andrewwutw/build-djgpp/releases/download/v2.9/"
+DJGPP_URL="https://github.com/andrewwutw/build-djgpp/releases/download/v3.0/"
+if [ "$(uname)" = "Darwin" ]; then
+    #Mac
+    DJGPP_FILE="djgpp-osx-$GCCVER.tar.bz2"
+    if [ -z "HINTS" ]; then
+        export HINTS=macOS.2020
+    fi
+elif [ "$(expr substr $(uname -s) 1 5)" = "Linux" ]; then
+    #Linux
+    DJGPP_FILE="djgpp-linux64-$GCCVER.tar.bz2"
+    if [ -z "$HINTS" ]; then
+        export HINTS=linux.2020
+    fi
+elif [ "$(expr substr $(uname -s) 1 10)" = "MINGW32_NT" ]; then
+    #mingw
+    DJGPP_FILE="djgpp-mingw-$GCCVER-standalone.zip"
+else
+    echo "No DJGPP release for you, sorry."
+    exit 1
+fi
+
+DJGPP_URL="$DJGPP_URL$DJGPP_FILE"
+
+# export
+
+if [ ! -d lib ]; then
+mkdir -p lib
+fi
+
+cd lib
+if [ ! -f "$DJGPP_FILE" ]; then
+   if [ "$(uname)" = "Darwin" ]; then
+        #Mac
+	curl -L $DJGPP_URL -o $DJGPP_FILE
+   else
+        wget --no-hsts "$DJGPP_URL"
+   fi
+fi
+
+if [ ! -d djgpp/i586-pc-msdosdjgpp ]; then
+    tar xjf "$DJGPP_FILE"
+    rm -f $DJGPP_FILE
+fi
+
+# DOS-extender for use with djgpp
+if [ ! -d djgpp/cwsdpmi ]; then
+    if [ "$(uname)" = "Darwin" ]; then
+      	#Mac
+	curl http://sandmann.dotster.com/cwsdpmi/csdpmi7b.zip -o csdpmi7b.zip
+    else
+	wget --no-hsts http://sandmann.dotster.com/cwsdpmi/csdpmi7b.zip
+    fi
+    cd djgpp
+    mkdir -p cwsdpmi
+    cd cwsdpmi
+    unzip ../../csdpmi7b.zip
+    cd ../../
+    rm csdpmi7b.zip
+fi
+
+#  PDCurses
+if [ ! -d "pdcurses" ]; then
+	echo "Getting ../pdcurses from https://github.com/wmcbrine/PDCurses.git" ; \
+	git clone --depth 1 https://github.com/wmcbrine/PDCurses.git pdcurses
+fi
+
+cd ../
+
+# Don't fail the build if lua fetch failed because we cannot do anything about it
+# but don't bother proceeding forward either
+if [ ! -d "lib/lua-$LUA_VERSION/src" ]; then
+        exit 0
+fi
+