C++ runtime should not be hardcoded in configure

[barracuda156]

This will be wrong for most set-up of macOS:

# Establish -lstdc++ or equivalent
if test "x$CXXLIB" == "x"; then
   if test "x$CXX" == "xifort"; then
      CXXLIB="-cxxlib"
   else
      CXXLIB="-lstdc++"
   fi
fi

As well as some other systems, which happen to use libc++ (but not ifort fortran).

[barracuda156]

UPD. Surprisingly, some runtime flag is still required for w/e reason: just trashing it breaks linking.

[jfowkes]

Indeed, this is why we are transitioning to Meson. The autotools build system was originally written only for Linux. I'ts tricky to do this correctly on Mac with autotools as by default gcc is a symlink to clang on macOS (who thought that was a good idea?).

[barracuda156]

Replacing the flag with -lc++ works for Clang builds. So if a test is added for the compiler being used, then it shall work correctly: use -lc++ when clang is being used (clang is never a symlink to gcc), otherwise use -lstdc++ for gcc.

[jfowkes]

Thanks @barracuda156, we can definitely do that.

[mjacobse]

use -lc++ when clang is being used (clang is never a symlink to gcc), otherwise use -lstdc++ for gcc.

I'm not sure this is ideal either. On Linux it is perfectly normal to use clang with libstdc++. I'd say on most distributions that is the default. Usually libc++ is even a seperate package, so you may not even have libc++ installed when using clang, which will break with that rule above.

I think ideally autotools would figure out which C++ standard library is used when linking with CXX and link that one explicitly when linking with FC. Or at least look for any standard library that's available and link whichever is found first, possibly configurable by the user.

[barracuda156]

@mjacobse It is also possible to use gcc with libc++, though it is not a typical case.

Anyway, the patch so far does not work for Clang anyway. And I agree with you, the right thing is to test for C++ runtime being used, not compiler. Going by compiler is an acceptable heuristic fixing typical scenarios, but leaving some other cases broken.

[barracuda156]

For the record, with the right library being set (as of now, we just patch it in for Clang builds), spral build fine on every macOS version: https://ports.macports.org/port/spral/details
(PowerPC being tested locally by myself, the rest we see via buildbots here.)

[jfowkes]

Yeah I agree that ideally we should test for the C++ runtime but I have no idea if that is even possible with autotools let alone how to do it.

[barracuda156]

@jfowkes Maybe solve it another way: why at all an explicit flag is needed at the moment? It is needed indeed, but it should not be. Normally you never have to pass this flag manually, not on macOS at least.

[jfowkes]

@barracuda156 that is a good point but I believe it is mandatory when the Fortran compiler is used to link everything together at the end (cf your link failures in #170). SPRAL is very unusual in its mixing of Fortran and C++.