[patch, fortran] C prototype writing improvements for gfortran

[Thomas Koenig]

Hello world,

the attached patch fixes PR 90351 (not all prototypes were written
to standard output with -fc-prototypes) and introduces new
functionality to also write C prototypes for external functions,
at the same time discouraging their use (because BIND(C) is really
the better, standard-conforming and portable way).  While looking
at the code, I also noticed that COMPLEX was not handled before,
so I added that, too.

Example:

$ cat c.f90
integer function r(i)
end

subroutine foo(a,b,c)
   character*(*) a
   real b
   complex c
end

character*(*) function x(r, c1,c2)
   real r
   character*(*) c1,c2
end
$ gfortran -fsyntax-only -fc-prototypes-external c.f90
/* Prototypes for external procedures generated from c.f90
    by GNU Fortran (GCC) 10.0.0 20190427 (experimental).

    Use of this interface is dicsouraged, consider using the
    BIND(C) feature of standard Fortran instead.  */

int r_ (int *i);
void foo_ (char *a, float *b, float complex *c, size_t a_len);
void x_ (char *result_x, size_t result_x_len, float *r, char *c1, char 
*c2, size_t c1_len, size_t c2_len);

I'd like to commit this to trunk and to gcc-9, to help users of
old-fashioned Lapack bindings, such as R, with their transition
to something that does not violate gfortran's ABI.

Tested with "make dvi" and "make info".  Otherwise, since these flags
are not tested in the testsuite (maybe they should be, I just don't
know how), regression test passed.

OK?

2019-05-08  Thomas Koenig  <tkoenig@gcc.gnu.org>

         PR fortran/90351
         PR fortran/90329
         * gfortran.dg/dump-parse-tree.c: Include version.h.
         (gfc_dump_external_c_prototypes): New function.
         (get_c_type_name): Select "char" as a name for a simple char.
         Adjust to handling external functions. Also handle complex.
         (write_decl): Add argument bind_c. Adjust for dumping of external
         procedures.
         (write_proc): Likewise.
         (write_interop_decl): Add bind_c argument to call of write_proc.
         * gfortran.h: Add prototype for gfc_dump_external_c_prototypes.
         * lang.opt: Add -fc-prototypes-external flag.
         * parse.c (gfc_parse_file): Move dumping of BIND(C) prototypes.
         Call gfc_dump_external_c_prototypes if option is set.
         * invoke.texi: Document -fc-prototypes-external.


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