many -Wincompatible-pointer-types on clang with gdb-7.9

[Jack Howarth]

On gdb-7.9, we are seeing a large number of warnings from
-Wincompatible-pointer-types with clang...

./guile/guile.c:474:25: warning: incompatible pointer types
initializing 'scm_t_subr' (aka 'void (*)()') with an expression of
type 'SCM (SCM, SCM)'
      [-Wincompatible-pointer-types]
  { "execute", 1, 0, 1, gdbscm_execute_gdb_command,
                        ^~~~~~~~~~~~~~~~~~~~~~~~~~
./guile/guile.c:487:32: warning: incompatible pointer types
initializing 'scm_t_subr' (aka 'void (*)()') with an expression of
type 'SCM (void)'
      [-Wincompatible-pointer-types]
  { "data-directory", 0, 0, 0, gdbscm_data_directory,
                               ^~~~~~~~~~~~~~~~~~~~~
./guile/guile.c:491:38: warning: incompatible pointer types
initializing 'scm_t_subr' (aka 'void (*)()') with an expression of
type 'SCM (void)'
      [-Wincompatible-pointer-types]
  { "guile-data-directory", 0, 0, 0, gdbscm_guile_data_directory,
                                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~

These all are associated with the guile/scm-*.c files.
           Jack