max_align_t fails with clang 6.1 on OS X 10.10

[Werner LEMBERG]

Building ttfautohint (http://repo.or.cz/ttfautohint.git/) on OS X
10.10 (XCode 6.4) fails.  The data below is taken from a travis build
(https://travis-ci.org/source-foundry/ttfautohint-build/jobs/269031319#L3401).

Here's the list of the gnulib modules I use in ttfautohint:

  dirname
  fcntl-h
  getopt-gnu
  git-version-gen
  isatty
  memmem-simple
  progname
  stdarg  
  stdbool 
  stdint  
  strerror_r-posix
  strndup
  strtok_r
  strtoull
  vasprintf


    Werner


======================================================================

  ...
  checking for max_align_t... no
  ...

  In file included from ../gnulib/src/stdio.h:53:
  ../gnulib/src/stddef.h:106:3: In file included from main.cpp:error26: :
  In file included from ../gnulib/src/stdio.htypedef redefinition with 
different types ('union max_align_t' vs 'long double'):53
:
  ../gnulib/src/stddef.h:106:3: error: typedef redefinition with different 
types ('union max_align_t' vs 'long double')
  } max_align_t;
    ^
  } max_align_t;
  
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/clang/6.1.0/include/stddef.h
  ^:
  119:21: note: previous definition is here
  
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/clang/6.1.0/include/stddef.h:119:21:
 note: previous definition is here
  typedef long double max_align_t;
                    ^
  typedef long double max_align_t;
                    ^
  1 error generated.

[there are obviously two output streams intermingled in the log file;
 I haven't cleaned up the messages]

Note that ttfautohint can be compiled fine with OS X 10.11 and newer;
it also succeeds on OS X 10.5.8 (which still uses gcc by default).


    Werner