gdb/: Require a C++ compiler

gdb/: Require a C++ compiler

This removes all support for building gdb & gdbserver with a C
compiler from gdb & gdbserver's build machinery.

gdb/ChangeLog:
2016-09-05  Pedro Alves  <palves@redhat.com>

	* NEWS: Mention that a C++ compiler is now required.
	* Makefile.in (COMPILER, COMPILER_CFLAGS): Remove.
	(COMPILE.pre, CC_LD): Use CXX directly.
	(INTERNAL_CFLAGS_BASE): Use CXXFLAGS directly.
	* acinclude.m4: Don't include build-with-cxx.m4.
	* build-with-cxx.m4: Delete file.
	* configure.ac: Remove GDB_AC_BUILD_WITH_CXX call.
	* warning.m4: Assume $enable_build_with_cxx is yes.
	* configure: Regenerate.

gdb/gdbserver/ChangeLog:
2016-09-05  Pedro Alves  <palves@redhat.com>

	* Makefile.in (COMPILER, COMPILER_CFLAGS): Remove.
	(COMPILE.pre, CC_LD): Use CXX directly.
	(INTERNAL_CFLAGS_BASE): Use CXXFLAGS directly.
	* acinclude.m4: Don't include build-with-cxx.m4.
	* configure.ac: Remove GDB_AC_BUILD_WITH_CXX call.
	* configure: Regenerate.