[patch] Extend AC_PROG_CXX to check for standards conformance

[Roger Leigh]

Patch attached.  Probably not in its final form, but I hope
it will be useful as a basis for discussion.

This is like the existing C89/99/11 checks, but for C++.
In this patch, I've implemented three checks:

AC_PROG_CXX_CXX98
AC_PROG_CXX_CXX98TR1
AC_PROG_CXX_CXX11

and AC_PROG_CXX will try them in the order
  AC_PROG_CXX_CXX11, AC_PROG_CXX_CXX98TR1, AC_PROG_CXX_CXX98
in order to put the compiler in the highest available mode
by default.  Each one performs a set of language and/or
library tests for each standard version.

Note that like the C equivalents, these are marked deprecated
and are not usable in their own right.  I'm still of the opinion
that it's useful to restrict the compiler to a minimum standard
so that you can
- prevent the use of features you don't want to use
- ensure that the features you do want are present
So ideally, I'd like to re-enable the C and C++ standard-
specific checks, if the consensus is that these remain useful,
and also add AC_PROG_CC_C11, which is currently missing.

Other points for discussion:
- The flags for non-GCC compilers were obtained from online
  documentation.  They are untested, and may be wrong; they
  are certainly not complete since I don't use non-GCC
  compilers myself.  Fairly trivial to test and update if
  anyone has access to such compilers.
- The feature tests are restricted to those which pass with
  GCC 4.7.  Depending upon which other compilers we want to
  put into "C++11 mode", this test set may need reducing to
  the common set which are usable across different compilers
  and/or compiler versions.

I've not updated the manual yet; I'll do this if people are
generally happy with things.


Kind regards,
Roger

-- 
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0001-AC_PROG_CXX-Add-checks-for-C-11-C-98TR1-and-C-98.patch
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