gdb: Respect CXXFLAGS when building with C++ compiler

gdb: Respect CXXFLAGS when building with C++ compiler

Currently, even when built with --enable-build-with-cxx, gdb uses
CFLAGS instead of CXXFLAGS.  This commit fixes it.

CXXFLAGS set in the environment when configure was run is now honored
in the generated gdb/Makefile, and you can also override CXXFLAGS in
the command like at make time, with the usual 'make CXXFLAGS="..."'

Objects built with a C compiler (e.g., gnulib) still honor CFLAGS
instead.

gdb/ChangeLog:
2016-01-21  Pedro Alves  <palves@redhat.com>

	* Makefile.in (COMPILER_CFLAGS): New.
	(CXXFLAGS): Get it from configure.
	(INTERNAL_CFLAGS_BASE, INTERNAL_LDFLAGS): Use COMPILER_CFLAGS
	instead of CFLAGS.
	* build-with-cxx.m4 (GDB_AC_BUILD_WITH_CXX): Set and AC_SUBST
	COMPILER_CFLAGS.
	* configure: Regenerate.

gdb/gdbserver/ChangeLog:
2016-01-21  Pedro Alves  <palves@redhat.com>

	* Makefile.in (COMPILER_CFLAGS, CXXFLAGS): New.
	(INTERNAL_CFLAGS_BASE): Use COMPILER_CFLAGS instead of CFLAGS.
	* configure: Regenerate.