make: fix GCC warnings

*** 37,47 ****
  #include <avo/intl.h>   /* for NOCATGETS */
  
  typedef struct {
          short           init;
          pathpt          vector;
!         char            *env_var;
  } vroot_patht;
  
  typedef struct {
          vroot_patht     vroot;
          vroot_patht     path;
--- 37,47 ----
  #include <avo/intl.h>   /* for NOCATGETS */
  
  typedef struct {
          short           init;
          pathpt          vector;
!         const char      *env_var;
  } vroot_patht;
  
  typedef struct {
          vroot_patht     vroot;
          vroot_patht     path;
*** 57,67 ****
          { 0, NULL, NOCATGETS("VIRTUAL_ROOT")},
          { 0, NULL, NOCATGETS("PATH")},
          "", NULL, NULL, NULL, 0, 1};
  
  void
! add_dir_to_path(register char *path, register pathpt *pointer, register int position)
  {
          register int            size= 0;
          register int            length;
          register char           *name;
          register pathcellpt     p;
--- 57,67 ----
          { 0, NULL, NOCATGETS("VIRTUAL_ROOT")},
          { 0, NULL, NOCATGETS("PATH")},
          "", NULL, NULL, NULL, 0, 1};
  
  void
! add_dir_to_path(const char *path, register pathpt *pointer, register int position)
  {
          register int            size= 0;
          register int            length;
          register char           *name;
          register pathcellpt     p;
*** 105,121 ****
                          if (p) *p= ':';
                          else return(result);};
          return((pathpt)NULL);
  }
  
! char *
  get_vroot_name(void)
  {
          return(vroot_data.vroot.env_var);
  }
  
! char *
  get_path_name(void)
  {
          return(vroot_data.path.env_var);
  }
  
--- 105,121 ----
                          if (p) *p= ':';
                          else return(result);};
          return((pathpt)NULL);
  }
  
! const char *
  get_vroot_name(void)
  {
          return(vroot_data.vroot.env_var);
  }
  
! const char *
  get_path_name(void)
  {
          return(vroot_data.path.env_var);
  }