scons: Disallow undefined symbols in Xlib libGL.so.

It's not the first time that, due to missing build dependencies or
incomplete commits, we end up with a broken libGL.so that's missing
symbols, causing all tests to fail catastrophically.

Instead try to catch this sort of issues earlier.

src/gallium/targets/libgl-xlib/SConscript

@@ -45,6 +45,9 @@ if env['llvm']:
     env.Append(CPPDEFINES = ['GALLIUM_LLVMPIPE'])
     env.Prepend(LIBS = [llvmpipe])
 
+# Disallow undefined symbols
+env.Append(SHLINKFLAGS = ['-Wl,-z,defs'])
+
 # libGL.so.1.5
 libgl_1_5 = env.SharedLibrary(
     target ='GL',