Fix a -Wformat bug exposed by compiling with Clang.
To preprocess C++11 code, you need `g++ -E -std=c++11`, not just
`g++ -E`. (The old code worked for GCC 6+ and Clang 6+ because they
changed the default mode from C++03 to C++14. But for GCC 5, we still
need `-std=c++11`. And regardless, it's a good idea.)
Add a "-mac" option to mymake, and cleanly factor out `set_mac`,
`set_linux`, and `set_win`. When you build mymake using
`make -f Makefile.simple mymake`, you get a mymake that knows what
platform it's on. This means you don't have to pass `mymake -mac`
on OSX, nor `mymake -win` on Windows.
The old code put `INCLUDE(___hyper-main.cpp)` into a C++ file that
would be preprocessed, which doesn't work because libSDL does
essentially `-Dmain=SDL_main`, which turns this into
`INCLUDE(___hyper-SDL_main.cpp)`, which gives us
a "file not found" error from mymake. The solution is to put
filenames into quotation marks, so that the string "main" never
appears as a token in the C++ file. (Alternatively, we could have
renamed "hyper-main.cpp" to "hypermain.cpp".)
Add several new "mymake" entries in the Travis build matrix,
and add the "mymake" builds' badge to the README.
