refine C++ header checking compiler invocation

refine C++ header checking compiler invocation

g++ 4.1.x dies with "cc1plus: error: output filename specified twice"
on the currently used construct. That's apparently due to it converting
the manually specified "c++" into "c++-header", and mis-handling that
(which, when using "c++-header" explicitly btw gets mis-handled even
with 4.9.x and also, using "c-header", by the plain C compiler).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Tim Deegan <tim@xen.org>