I'm building git-master versions of X and mesa on an x86_64 box running
slackware.   I've been doing this for a long time.  My build fails with:

 CC     nouveau_screen.lo
nouveau_screen.c: In function ‘nouveau_init_screen2’:
nouveau_screen.c:105:8: error: implicit declaration of function
‘nouveau_drm_new’ [-Werror=implicit-function-declaration]
  ret = nouveau_drm_new(dri_screen->fd, &screen->drm);
        ^~~~~~~~~~~~~~~
nouveau_screen.c:111:8: error: implicit declaration of function
‘nouveau_device_new’ [-Werror=implicit-function-declaration]
  ret = nouveau_device_new(&screen->drm->client, NV_DEVICE,
        ^~~~~~~~~~~~~~~~~~
nouveau_screen.c:111:39: error: dereferencing pointer to incomplete type
‘struct nouveau_drm’
  ret = nouveau_device_new(&screen->drm->client, NV_DEVICE,
                                       ^~
nouveau_screen.c: In function ‘nouveau_destroy_screen’:
nouveau_screen.c:230:2: error: implicit declaration of function
‘nouveau_drm_del’ [-Werror=implicit-function-declaration]
  nouveau_drm_del(&screen->drm);
  ^~~~~~~~~~~~~~~

I;ve got the current git/master of xf86-video-nouveau installed.