cl/gcc-adapt: Support for using g++ for debugging added.

'cc1' is still used by default. Use the 'run-test-cc1plus' script for
running g++ with the Code Listener.
cl/llvm: fix a compile-time warning

cl/llvm/clplug.cc: In member function ‘void CLPass::handleCallInstruction(llvm::CallInst*)’:
cl/llvm/clplug.cc:1550:36: warning: ISO C++ forbids variable length array ‘argumets’ [-Wvla]
     struct cl_operand argumets[args];
                                    ^
switch-host-gcc.sh: mention compatiblity patches

... for ancient GCC_HOST versions

reported-by: David Kašpar