Make Sage build with clang (3.7+) and make it the default on OS X

[ohanar]

Apple stopped supporting GCC in favour of clang. This is to track the status of building Sage with clang with the aim of making it the default compiler used on OS X and other OS where it is the default compiler.

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Packages failing their tests

• eclib, reported upstream at eclib test suite fails when compiled with clang due to tolerance problems JohnCremona/eclib#19
• giac - patch to pass tests with gcc/pari-2.8+ causes failures with clang fixed by Upgrade giac to 1.4.9-45 #24174
• ccache ccache fails test suite with clang #22836

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Supporting clang not only on OSX:

• porting to clang/FreeBSD -- port Sage to FreeBSD 11 #22679

Depends on #24579

CC: @nexttime @jpflori @dimpase @mkoeppe @isuruf

Component: build

Keywords: clang porting

Author: François Bissey

Branch: ac7816d

Reviewer: Jeroen Demeyer, John Palmieri, Dima Pasechnik

Issue created by migration from https://trac.sagemath.org/ticket/12426

[jdemeyer]

comment:7

I guess many of these tickets are useful for porting in general.

[nexttime]

Changed keywords from clang to clang C++11 porting

[nexttime]

comment:13

Added further ticket references.

[nexttime]

comment:15

Description presumably needs some updating...

[nexttime]

comment:22

eclib has been fixed upstream last year (IIRC) as well; currently re-checking whether our current version still builds...

[vbraun]

comment:483

Is installed and matches version

buildbot@sagebd07_64s02:~$ gcc --version
gcc (Debian 4.7.2-5) 4.7.2
[...]
buildbot@sagebd07_64s02:~$ gfortran --version
GNU Fortran (Debian 4.7.2-5) 4.7.2

[vbraun]

comment:484

• The error happend with both incremental and full build
• Only happens on Debian 7, not other distros

[kiwifb]

comment:485

The bit that's supposed to prevent this from happening in configure is

# if GCC needs to be installed GFORTRAN shouldn't be installed
# GCC will provide GFORTRAN in that case
if test $need_to_install_gcc = yes ; then
    need_to_install_gfortran=no
fi

so this test must be failing with the shell used debian 7. It will to be converted into something more universally reliable.

[dimpase]

comment:486

Could it be that on that Debian 7 system /bin/sh is not bash? This might explain this singularity.

[jdemeyer]

comment:487

Please read [comment:479] for a possible explanation and see #24579 for a possible fix...

[dimpase]

comment:488

Why is this in sage-pending?

[vbraun]

Changed branch from u/jdemeyer/clang-progress to ac7816d

[kiwifb]

comment:491

Thanks everyone!

[kiwifb]

Changed commit from ac7816d to none