SCons: Fix usage of LD when we meant LINK

Also made LINK and CXXFLAGS configurable as command line options.
Note that LINK currently expects the *compiler* that will be used
for linking and will call its configured linker behind the scenes
(so g++, clang++, etc., not ld.gold). See #15364 for details.

platform/server/detect.py

@@ -59,7 +59,7 @@ def configure(env):
         if ('clang++' not in env['CXX']):
             env["CC"] = "clang"
             env["CXX"] = "clang++"
-            env["LD"] = "clang++"
+            env["LINK"] = "clang++"
         env.Append(CPPFLAGS=['-DTYPED_METHOD_BIND'])
         env.extra_suffix = ".llvm" + env.extra_suffix