fix interaction of -S and -x {c,c++}-header

fix interaction of -S and -x {c,c++}-header

Irrespective of the use of -o this so far resulted in "error: output
filename specified twice", since cc1_options already produces a -o
option when -S was specified.

gcc/
2016-07-04  Jan Beulich  <jbeulich@suse.com>

	* gcc.c (default_compilers["@c-header"]): Conditionalize "-o".

gcc/cp/
2016-07-04  Jan Beulich  <jbeulich@suse.com>

	* lang-specs.h ("@c++-header"): Conditionalize "-o".

gcc/testsuite/
2016-07-04  Jan Beulich  <jbeulich@suse.com>

	* g++.dg/header.c: New.
	* gcc.dg/header.c: New.

From-SVN: r237955