scons: Force C++ linkage.

We have some C++ code (LLVM), which must be linked with g++ on certain
platforms. SCons tries to guess when to use g++ by looking to the source files,
but this fails, if the C++ code is not list in the command line, but inside in
the library.

SConstruct

@@ -206,7 +206,10 @@ if llvm:
 	env.ParseConfig('llvm-config --cflags --ldflags --libs')
 	env.Append(CPPDEFINES = ['MESA_LLVM'])
 	env.Append(CXXFLAGS = ['-Wno-long-long'])
-	
+
+# Force C++ linkage
+if env['PLATFORM'] in ('posix',):
+	env['LINK'] = env['CXX']
 
 # libGL
 if platform not in ('winddk',):