[BSD] include paths (/usr/local/include).

Hello world :-)

I have tried to build the `sim:nxwm` example, but got this on FreeBSD:

```
./sim/posix/sim_x11framebuffer.c:29:10: fatal error: 'X11/Xlib.h' file not found
#include
^~~~~~~~~~~~
1 error generated.
```

This file exist but in the `/usr/local` include prefix:

```
% ls -al /usr/local/include/X11/Xlib.h
-rw-r--r-- 1 root wheel 99811 1 gru 02:14 /usr/local/include/X11/Xlib.h
```

What would be the best way to add this path to global includes? :-)