The X11 constants are different between versions.
For example, Debian doesn't have XF86XK_WWAN
https://salsa.debian.org/xorg-team/proto/x11proto-core/blob/debian-unstable/XF86keysym.h
But Arch Linux does.
https://www.archlinux.org/packages/extra/any/xorgproto/
I've got following error on Debian stretch
```
/home/sei/src/github.com/k0kubun/xremap/src/x11_constants.c:2710:20: error: ‘XF86XK_WWAN’ undeclared (first use in this function)
define_x11_const(XF86XK_WWAN);
```
To avoid macro undeclared error, wrap the `define_x11_const` with #ifdef
```
#ifdef XF86XK_WWAN
define_x11_const(XF86XK_WWAN)
#endif
```
Or generate code at build-time.
