scons: Don't use -z defs ld option on Mac.

Should fix fdo bug 67098.

src/gallium/targets/libgl-xlib/SConscript

@@ -46,7 +46,8 @@ if env['llvm']:
     env.Prepend(LIBS = [llvmpipe])
 
 # Disallow undefined symbols
-env.Append(SHLINKFLAGS = ['-Wl,-z,defs'])
+if env['platform'] != 'darwin':
+    env.Append(SHLINKFLAGS = ['-Wl,-z,defs'])
 
 # libGL.so.1.5
 libgl_1_5 = env.SharedLibrary(

src/mesa/drivers/x11/SConscript

@@ -32,7 +32,8 @@ sources = [
 ]
 
 # Disallow undefined symbols
-env.Append(SHLINKFLAGS = ['-Wl,-z,defs'])
+if env['platform'] != 'darwin':
+    env.Append(SHLINKFLAGS = ['-Wl,-z,defs'])
 
 # libGL.so.1.6
 libgl_1_6 = env.SharedLibrary(