libtool: Add support for flang (Fortran LLVM-based) compilers
This patch adds support for flang compilers. Some specific flags
are needed so these compilers must be handled in a specific way.
By default, the compiler is called 'flang', but ARM releases their
own flang-based compiler called 'armflang'.
Because of the current lack of flang support in libtool, the
generated 'libtool' must be manually modified after 'configure' is
invoked. Such a process is for example described on ARM web site
(it involves the Open MPI library).