Add GNU/Hurd as platform

SDL has been building on GNU/Hurd for a long time, using either drivers
based on external libraries (e.g. X11, pulseaudio, sndio, etc) or dummy
drivers. This commit introduces it explicitly as platform, so it can be
recognized, and tweaked as needed. In particular:
- introduce the SDL_PLATFORM_HURD define
- tighten/improve the platform detection in cmake, and use "Hurd" as
  identifier
- return the platform name in SDL_GetPlatform()
- tweak the CFLAGS/LDFLAGS so pthreads can be used properly
- implement SDL_GetExeName(), using /proc/self/exe as provided by the
  basic Linux-like procfs
- enable GLES 2 in tests (mostly for consistency with Linux)

src/SDL.c

@@ -770,6 +770,8 @@ const char *SDL_GetPlatform(void)
     return "PlayStation Vita";
 #elif defined(SDL_PLATFORM_3DS)
     return "Nintendo 3DS";
+#elif defined(SDL_PLATFORM_HURD)
+    return "GNU/Hurd";
 #elif defined(__managarm__)
     return "Managarm";
 #else