New compiler warnings

[Gerald Pfeifer]

My nightly autobuilder on FreeBSD 5.1 noticed the following warning
introduced yesterday:

signal_i386.c:524: warning: `save_context' defined but not used
  signal_i386.c:602: warning: `restore_context' defined but not used

Also, there are two OpenGL warnings left, after most have been fixed
a few months ago:

directx.c:852: warning: implicit declaration of function `glXGetProcAddressARB'
  drawprim.c:829: warning: implicit declaration of function `glMultiTexCoord4fARB'

Could you please have a look? Thanks!

Gerald