Support for Flang and ARM HPC compiler

Hello all,

This patch is a part of the cross-project set of patches
that soon will be sent to all of the relevant GNU Autotools
components: gnulib, autoconf, automake and libtool.

The purpose of this is to add awareness of the new LLVM Fortran
compiler (Flang) along with LLVM-derived ARM HPC Compilers.

These new compilers are recognized by their executable binary
names: flang, armflang, armclang, armclang++

Without those patches 'configure' script generates wrong set
of command-line flags.

This is a re-send of the patch after rebase.

Best regards,
Paul Osmialowski