[docs] add some mxe pointers

docs/build-static.rst

+some notes to build the gui with static qt5
+
 QMAKE=/toolchain/qt5-static/bin/qmake TARGET=demolib-vpn ./build.sh

docs/cross-compiling-with-mxe.rst

+some attempts at mxe cross-compilation
+======================================
+
+We really would like to have cross-compilation working.
+
+I tried mxe, it looks the most promising way to, at least, get the binaries
+working. (Cross-compiling a static version of QtIFW might prove more difficult,
+though).
+
+these two links were useful for me in my attempts:
+
+https://gist.github.com/amitsaha/ec8fbbc01e22ef9cc020570f415fa2fb
+https://stackoverflow.com/questions/14170590/building-qt-5-on-linux-for-windows
+
+I tried the mxe project stretch packages
+
+* debs seem to be broken :(
+* add this repo::
+
+  deb http://pkg.mxe.cc/repos/apt stretch main
+
+* install this package::
+
+  mxe-x86_64-w64-mingw32.static-qt5
+
+
+- I think I tried with cmake. should try again now that I went the qmake route.
+- Compiling things with the instructions above got me further. However, I only compiled a very simple qt app - did not try with all the QML/foo libraries. It should not be much harder...
+- I had to patch some files in mxe to workaround a couple of issues (basically editing include paths). TODO -- dig those patches and include them here.