make: fix GCC warnings

@@ -37,11 +37,11 @@
 #include <avo/intl.h>   /* for NOCATGETS */
 
 typedef struct {
         short           init;
         pathpt          vector;
-        char            *env_var;
+        const char      *env_var;
 } vroot_patht;
 
 typedef struct {
         vroot_patht     vroot;
         vroot_patht     path;

@@ -57,11 +57,11 @@
         { 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)
+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;

@@ -105,17 +105,17 @@
                         if (p) *p= ':';
                         else return(result);};
         return((pathpt)NULL);
 }
 
-char *
+const char *
 get_vroot_name(void)
 {
         return(vroot_data.vroot.env_var);
 }
 
-char *
+const char *
 get_path_name(void)
 {
         return(vroot_data.path.env_var);
 }