Update install instructions for gfortran
Don't warn about uninitialised in tests

Getting errors like

    pfunit/include/driver.F90: In function 'MAIN__':

    pfunit/include/driver.F90:34:0: error: '.executable' may be used
    uninitialized in this function [-Werror=maybe-uninitialized]

    executable = getCommandLineArgument(0)