Re: max_align_t fails with clang 6.1 on OS X 10.10

[Werner LEMBERG]

>> configure:17517: gcc -c
>> -I/Users/travis/ttfautohint-build/local/include -g -O2 conftest.c >&5
>> conftest.c:145:13: error: use of undeclared identifier 'max_align_t'
> 
> Evidently the stddef.h found by the above command does not define
> max_align_t, whereas the stddef.h found by the command 'CXX main.o'
> in
> <https://travis-ci.org/source-foundry/ttfautohint-build/jobs/269031319#L3401>
> does define max_align_t.  Can you investigate why this is so?
> Perhaps CXX uses different -I options, or perhaps stddef.h does not
> define max_align_t if C++.

I suspect that the used clang version doesn't activate C11 support by
default, while C++11 is available.

Right now I only have the `stddef.h' file of a later XCode version
(from 2015); however, looking at the copyright notice it seems that it
is unchanged.  It contains the following.

  #if (defined (__STDC_VERSION__) && __STDC_VERSION__ >= 201112L) \
      || (defined(__cplusplus) && __cplusplus >= 201103L)
  typedef long double max_align_t;
  #endif

Chris, do you have a chance to check

  
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/clang/6.1.0/include/stddef.h

of an OS X 10.10 Travis run whether the definition is identical?

> Ideally 'configure' should be using the same compiler and options as
> 'make'; when that isn't true, there can be trouble.

Sounds sensible.  However, how to do that correctly?  The
`max_align_t' test is part of gnulib, I don't ask for it.  Shall I
simply add

  AC_LANG_PUSH([C++])

right before the call to

  gl_INIT

in my `configure.ac' file?  Note that I also have

  AX_CXX_COMPILE_STDCXX([11],,[optional])

to activate C++11 if available.


    Werner