scons: Add Haiku build support

Enables building stock Mesa under the Haiku operating system.

src/gallium/SConscript

@@ -10,16 +10,22 @@ SConscript('auxiliary/SConscript')
 # Drivers
 #
 
+# These are common and work across all platforms
 SConscript([
     'drivers/galahad/SConscript',
     'drivers/identity/SConscript',
-    'drivers/llvmpipe/SConscript',
     'drivers/rbug/SConscript',
     'drivers/softpipe/SConscript',
-    'drivers/svga/SConscript',
     'drivers/trace/SConscript',
 ])
 
+# These drivers do not build on Haiku
+if env['platform'] not in ['haiku']:
+    SConscript([
+        'drivers/llvmpipe/SConscript',
+        'drivers/svga/SConscript',
+    ])
+
 if not env['msvc']:
     # These drivers do not build on MSVC compilers
     SConscript([
@@ -53,7 +59,7 @@ SConscript('winsys/sw/null/SConscript')
 
 if not env['embedded']:
     SConscript('state_trackers/vega/SConscript')
-    if env['platform'] not in ['darwin']:
+    if env['platform'] not in ['darwin', 'haiku']:
         SConscript('state_trackers/egl/SConscript')
 
     if env['x11']:
@@ -83,7 +89,7 @@ SConscript([
 ])
 
 if not env['embedded']:
-    if env['platform'] not in ['darwin']:
+    if env['platform'] not in ['darwin', 'haiku']:
         SConscript([
             'targets/egl-static/SConscript'
         ])