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.

rogueviz/platformer.cpp

@@ -375,7 +375,7 @@ static double gtime = 0;
 
 double vel_x = 0, vel_y = 0;
 
-static const double grav = 0.1;
+static constexpr double grav = 0.1;
 
 bool map_on = false;