Oops. A change to an upstream header seems to have remove glu.h, which

these files require.  Dunno why my build didn't pick up the transitive
dependency.

src/ATC/ATCdisplay.cxx

@@ -22,6 +22,8 @@
 #  include <config.h>
 #endif
 
+#include <GL/glu.h>
+
 #include <simgear/props/props.hxx>
 
 #include <Include/general.hxx>

src/Cockpit/cockpit.cxx

@@ -29,6 +29,8 @@
 #  include <windows.h>
 #endif
 
+#include <GL/glu.h>
+
 #include <stdlib.h>
 #include <stdio.h>
 #include <string.h>

src/Cockpit/hud.cxx

@@ -44,6 +44,8 @@
 #include <stdio.h>		// char related functions
 #include <string.h>		// strcmp()
 
+#include <GL/glu.h>
+
 #include <simgear/constants.h>
 #include <simgear/debug/logstream.hxx>
 #include <simgear/props/props.hxx>

src/Cockpit/panel.cxx

@@ -29,6 +29,8 @@
 #include <stdio.h>	// sprintf
 #include <string.h>
 
+#include <GL/glu.h>
+
 #include <plib/ssg.h>
 #include <plib/fnt.h>