Change static const to static constexpr wherever possible

Since we require C++11, most of these consts can be constexpr.

Two `static const ld` remain non-compile-time-evaluable because
they depend on the runtime `log` function. One `static const cld`
remains non-compile-time because `std::complex<T>` doesn't become
constexpr until C++14.

control.cpp

@@ -34,7 +34,7 @@ EX function <bool(SDL_Event &ev)> joyhandler = [] (SDL_Event &ev) {return false;
 
 #if HDR
 // what part of the compass does 'skip turn'
-static const auto SKIPFAC = .4;
+static constexpr auto SKIPFAC = .4;
 #endif
 
 // is the player using mouse? (used for auto-cross)