wflinfo: Fix glGetStringi on Mali

On Mali EGL 1.4, you can't obtain non-extension functions (including
OpenGL ES 3.0 functions) with eglGetProcAddress. You must use dlsym.

This patch carefully avoids breaking glGetStringi on other platforms,
which may expose glGetStringi dynamically (as in eglGetProcAddress) but
not statically (as in dlsym).

Fixes #20: https://github.com/waffle-gl/waffle/pull/20
Fixes: chromium:428061 [https://code.google.com/p/chromium/issues/detail?id=428061]
See: https://github.com/anholt/libepoxy/issues/21
Reported-by: Frank Henigman <fjhenigman@chromium.org>
Tested-by: Daniel Kurtz <djkurtz@chromium.org>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Chad Versace <chad.versace@linux.intel.com>