Support building Sage with the Flang Fortran compiler

[slel]

Part of:

• Meta-ticket: allow more Fortran compilers #23926: Meta-ticket: allow more Fortran compilers

CC: @dimpase @mkoeppe @slel

Component: build

Keywords: fortran, flang

Issue created by migration from https://trac.sagemath.org/ticket/32176

[slel]

comment:1

From a discussion at #30835 comment:11.

Please confirm we are talking about this Flang:

• http://flang.llvm.org/docs/

rather than this one:

• https://github.com/flang-compiler/flang

[slel]

Changed keywords from none to fortran, flang

[dimpase]

comment:2

The former is not yet a real compiler.

[williamstein]

comment:5

Replying to @dimpase:

The former is not yet a real compiler.

Can you elaborate?

[dimpase]

comment:6

Replying to @williamstein:

Replying to @dimpase:

The former is not yet a real compiler.

Can you elaborate?

https://releases.llvm.org/11.0.0/tools/flang/docs/ReleaseNotes.html says that it needs an "external Fortran compiler", defaulted to gfortran, to operate

Using Flang

Usage: flang hello.f90 -o hello.bin

By default, Flang will parse the Fortran file hello.f90 then unparse it to a canonical Fortran source file. 
Flang will then invoke an external Fortran compiler to compile this source file and link it, placing the resulting executable in hello.bin.

To specify the external Fortran compiler, set the F18_FC environment variable to the name of the compiler binary and ensure that it is on your PATH. 
The default value for F18_FC is gfortran.