cctools: try to use clang in as

[jmroot]

The clang integrated assembler is way more up-to-date than the ancient
pre-GPLv3 version of gas that is the alternative. The latter doesn't
know about newer instructions such as AVX. The as(1) shipped with
recent Xcode versions will just run clang behind the scenes under most
circumstances for this reason.

Our cctools can't actually depend on clang because that would create a
circular dependency. So we'll try to run the clang corresponding to the
LLVM being used by cctools, and if that isn't present, try the system
clang in /usr/bin if we're on an OS version that has a clang with a
working integrated assembler. (The ancient gas is still used as a final
fallback.)

Fixes: https://trac.macports.org/ticket/37846

Type(s)

• enhancement

Tested on

macOS 10.14
Xcode 10.1

[macportsbot]

Notifying maintainers:
@jeremyhu for port cctools.

[macportsbot]

Travis Build #5004 Passed.

Lint results

--->  Verifying Portfile for cctools
--->  0 errors and 0 warnings found.

Port cctools success on xcode9.4. Log
Port cctools success on xcode8.3. Log
Port cctools success on xcode7.3. Log
Port cctools success on xcode10.1. Log

[cjones051073]

LGTM... I like the way you try and find the correct MPs clang version, without directly depending on it, and fall back to trying /usr/bin/clang instead if not found.

With this patch I was successfully able to build OpenBLAS with AVX2 enabled, which currently has to be turned off due to the fact the symbols generated by gcc's fortran compiler could not be linked, due to the limitations of cctools.

https://trac.macports.org/ticket/57912

I think we should not sit on this for too long, and merge quickly.

@kencu What do you think ?

[jmroot]

I'll merge if @jeremyhu doesn't raise any objections by the end of the day.

[kencu]

a workable approach. The only other approach I could think of would be to have "as" respond to an env var with a clang path in it, and that seems more cumbersome. So +1 .

[cjones051073]

Its unlikely we will hear from @jeremyhu, if previous experience holds. I am happy with the changes here, so if no one else beats me to it, or objects, I am happy to be the one to hit go later on today...