[PROJ] PROJ 6.0.0RC3

[Even Rouault]

Greg,

> Here is the build failure.  It seems to be only mutex.cpp, so far.
> 
>   - I am unclear on all the -f and -W options and what they might cause.

I don't think they matter much here

> 
>   - There is no "--std=c++11".

That's a weird one. I do have g++ 5.5 on Ubuntu 16.04 and it defaults to
C++98, but ./configure automatically adds -std=c++11
But it would be strange that mutex.cpp is the only file to fail compilation.
C++11 is used extensively even in proj_internal.h, so all files should fail 
compilation.
Are you absolutely sure that your "c++" binary is g++ 5.5 ?

What does the following return ?
c++ -dM -E -x c++  /dev/null | grep -F __cplusplus

If that's indeed the only file to fail compilation, I'd suggest you try 
tweaking the start of the file to comment out the #define _XOPEN_SOURCE 500 
and/or #define _GNU_SOURCE

Even

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