Support Fortran 2003 bind(c), enum bind(c)

[raehik]

ICCS's fortran-tf-lib uses it for C interop: src/fortran_tensorflow_lib.F90#L24-L42

    enum, bind(c)
        enumerator :: TF_OK = 0
        enumerator :: TF_CANCELLED = 1
! ...
        enumerator :: TF_DATA_LOSS = 15
    end enum

Some short notes:

• enum, bind(c) is a nice simple block construct
• it might look flexible but enum, bind(c) must use that exact form. No plain enum.
• bind(c) is similar. No other bind(X).