`$FC` should allow the full path to compilers

[ScottWales]

Specifying the compiler's full path allows you to be explicit about which compiler you're using, without worrying about PATH ordering.

Using the full path to the compiler

FC=/usr/bin/gfortran

fab

Causes errors

Traceback (most recent call last):
  File "/home/swales/fab/source/fab/cli.py", line 65, in calc_linker_flags
    linker, linker_flags = linkers[fc]
                           ~~~~~~~^^^^
KeyError: '/usr/bin/gfortran'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/swales/miniconda3/envs/fab/bin/fab", line 8, in <module>
    sys.exit(cli_fab())
             ^^^^^^^^^
  File "/home/swales/fab/source/fab/cli.py", line 94, in cli_fab
    config = _generic_build_config(folder, kwargs)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/swales/fab/source/fab/cli.py", line 32, in _generic_build_config
    linker, linker_flags = calc_linker_flags()
                           ^^^^^^^^^^^^^^^^^^^
  File "/home/swales/fab/source/fab/cli.py", line 67, in calc_linker_flags
    raise NotImplementedError(f"Fab's zero configuration mode does not yet work with compiler '{fc}'")
NotImplementedError: Fab's zero configuration mode does not yet work with compiler '/usr/bin/gfortran'

• calc_linker_flags() should be looking at the basename of the compiler to decide what flags to add.
• calc_linker_flags() should consider getting the the linker from $LD or re-use $FC
• tools.COMPILERS should be indexed using the compiler's base name, or better still the compiler identity gotten from $FC --version to account for wrappers like mpifort