'make check-headers' for c++ header

'make check-headers' for c++ header

If I run 'make check-headers', I get these errors,
....
In file included from ../../binutils-gdb/gdb/common/common-defs.h:78:0,
                 from ../../binutils-gdb/gdb/defs.h:28,
                 from <command-line>:0:
../../binutils-gdb/gdb/common/common-utils.h:23:18: fatal error: string: No such file or directory
 #include <string>
                  ^

because we still parse headers as c file with a c compiler, which is no
longer true after we moved to C++.  This patch changes it to use C++
compiler and parse headers as c++ headers.

gdb:

2017-01-13  Yao Qi  <yao.qi@linaro.org>

	* Makefile.in (checker-headers): Use CXX and CXX_DIALET instead
	of CC.  Pass "-x c++-header" instead of "-x c".