Add --enable-build-with-cxx configure switch

Add --enable-build-with-cxx configure switch

This new option, disabled by default for now, allows specifying
whether to build GDB, GDBserver, and friends with a C++ (98/03)
compiler.

The name of the switch should be familiar to those who followed GCC's
own C++ conversion process.

. Adding -fpermissive to COMPILER in C++ mode (see the new
build-with-cxx.m4 file) makes errors like these be warnings instead:

  gdb/infrun.c:6597:1: error:   initializing argument 1 of ‘void sig_print_info(gdb_signal)’ [-fpermissive]
   sig_print_info (enum gdb_signal oursig)
   ^
  gdb/infrun.c: In function ‘void do_restore_infcall_suspend_state_cleanup(void*)’:
  gdb/infrun.c:7164:39: error: invalid conversion from ‘void*’ to ‘infcall_suspend_state*’ [-fpermissive]
     restore_infcall_suspend_state (state);
				 ^

so that the compiler carries on compiling the file.  -Werror still
catches the warnings, so nothing is lost, only our lifes are made
easier by concentrating on getting other more important things out of
the way first.

There's no way to quiet those warnings.  Until they're all fixed, when
building in C++ mode, -Werror is disabled by default.

. Adding -Wno-narrowing suppresses thousands of instances of this warning:

  gdb/arm-linux-tdep.c:439:1: error: narrowing conversion of ‘-1’ from ‘int’ to ‘ULONGEST {aka long unsigned int}’ inside { } is ill-formed in C++11 [-Werror=narrowing]
  gdb/arm-linux-tdep.c:439:1: error: narrowing conversion of ‘-1l’ from ‘LONGEST {aka long int}’ to ‘ULONGEST {aka long unsigned int}’ inside { } is ill-formed in C++11 [-Werror=narrowing]
  gdb/arm-linux-tdep.c:450:1: error: narrowing conversion of ‘-1’ from ‘int’ to ‘ULONGEST {aka long unsigned int}’ inside { } is ill-formed in C++11 [-Werror=narrowing]

We can defer handling those until we target C++11.

. Adding -Wno-sign-compare suppresses thousands of instances of this warning:

  gdb/linux-record.c:1763:32: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
	 if (tmpulongest == tdep->fcntl_F_GETLK64)
				  ^

. Adding -Wno-write-strings suppresses thousands of instances of this warning:

  gdb/mi/mi-cmd-var.c: In function ‘void mi_cmd_var_show_attributes(char*, char**, int)’:
  gdb/mi/mi-cmd-var.c:514:12: warning: deprecated conversion from string constant to ‘char*’ [-Wwrite-strings]
       attstr = "editable";
	      ^
  gdb/mi/mi-cmd-var.c:516:12: warning: deprecated conversion from string constant to ‘char*’ [-Wwrite-strings]
       attstr = "noneditable";
	      ^

For now, it's best to hide these warnings from view until we're
'-fpermissive'-clean, and can thus start building with -Werror.
The C compiler has always managed to build working GDBs with these
issues in the code, so a C++ compiler should too.

gdb/ChangeLog:
2015-02-27  Pedro Alves  <palves@redhat.com>

	* Makefile.in (COMPILER): New, get it from autoconf.
	(COMPILE.pre, CC_LD): Use COMPILER.
	(CXX): Get from autoconf instead.
	(CXX_FOR_TARGET): Default to g++ instead of gcc.
	* acinclude.m4: Include build-with-cxx.m4.
	* build-with-cxx.m4: New file.
	* configure.ac: Call AC_PROG_CXX and GDB_AC_BUILD_WITH_CXX.
	Disable -Werror by default if building in C++ mode.
	(build_warnings): Add -Wno-sign-compare, -Wno-write-strings and
	-Wno-narrowing in C++ mode.  Only enable -Wpointer-sign in C mode.
	Run supported-warning-flags tests with the C++ compiler.
	Save/restore CXXFLAGS too.
	* configure: Regenerate.

gdb/gdbserver/ChangeLog:
2015-02-27  Pedro Alves  <palves@redhat.com>

	* Makefile.in (COMPILER): New, get it from autoconf.
	(CXX): Get from autoconf instead.
	(COMPILE.pre): Use COMPILER.
	(CC-LD): Rename to ...
	(CC_LD): ... this.  Use COMPILER.
	(gdbserver$(EXEEXT), gdbreplay$(EXEEXT), $(IPA_LIB)): Adjust.
	(CXX_FOR_TARGET): Default to g++ instead of gcc.
	* acinclude.m4: Include build-with-cxx.m4.
	* configure.ac: Call AC_PROG_CXX and GDB_AC_BUILD_WITH_CXX.
	Disable -Werror by default if building in C++ mode.
	(build_warnings): Add -Wno-sign-compare, -Wno-write-strings and
	-Wno-narrowing in C++ mode. Run supported-warning-flags tests with
	the C++ compiler.  Save/restore CXXFLAGS too.
	* configure: Regenerate.