C++ : Add the -stdlib= option.

C++ : Add the -stdlib= option.

This option allows the user to specifiy alternate C++ runtime libraries,
for example when a platform uses libc++ as the installed C++ runtime.

We introduce the command line option: -stdlib= which is the user-facing
mechanism to select the C++ runtime to be used when compiling and linking
code.  This is the same option spelling as that used by clang to allow the
use of libstdc++.

The availability (and thus function) of the option are a configure-time
choice using the configuration control:
--with-gxx-libcxx-include-dir=

Specification of the path for the libc++ headers, enables the -stdlib=
option (using the path as given), default values are set when the path
is unconfigured.

If --with-gxx-libcxx-include-dir is given together with --with-sysroot=,
then we test to see if the include path starts with the sysroot and, if so,
record the sysroot-relative component as the local path.  At runtime, we
prepend the sysroot that is actually active.

At link time, we use the C++ runtime in force and (if that is libc++) also
append the libc++abi ABI library. As for other cases, if a target sets the
name pointer for the ABI library to NULL the G++ driver will omit it from
the link line.

gcc/ChangeLog:

	* configure.ac: Add gxx-libcxx-include-dir handled
	in the same way as the regular cxx header directory.
	* Makefile.in: Regenerated.
	* config.in: Likewise.
	* configure: Likewise.
	* cppdefault.c: Pick up libc++ headers if the option
	is enabled.
	* cppdefault.h (struct default_include): Amend comments
	to reflect the extended use of the cplusplus field.
	* incpath.c (add_standard_paths): Allow for multiple
	c++ header include path variants.
	* doc/invoke.texi: Document the -stdlib= option.

gcc/c-family/ChangeLog:

	* c.opt: Add -stdlib= option and enumerations for
	libstdc++ and libc++.

gcc/cp/ChangeLog:

	* g++spec.c (LIBCXX, LIBCXX_PROFILE, LIBCXX_STATIC): New.
	(LIBCXXABI, LIBCXXABI_PROFILE, LIBCXXABI_STATIC): New.
	(enum stdcxxlib_kind): New.
	(lang_specific_driver): Allow selection amongst multiple
	c++ runtime libraries.