Fix wrong behaviour of torad_coord() with gcc 8.1 -O2 (fixes #1084)

torad_coord() of gie.c has this sequence:
```
if( cond )
    axis = l->param + strlen ("axis=");
n = strlen (axis);
```

When the if branch is evaluated, n is always zero
even if on inspection axis is non empty

The reason is that the struct ARG_list which is the
type of l use a variable-length array for the param member

struct ARG_list {
    paralist *next;
    char used;
    char param[1];
};

But this is not a proper way of declaring it, and
gcc 8 has apparently optimizations to detect that l->param + 5
points out of the array, hence it optimizes strlen() to 0.

Reported as https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86914

According to https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html,
the proper way of declaring such arrays is to use [0]

src/pj_param.c

@@ -42,7 +42,7 @@ paralist *pj_mkparam_ws (char *str) {
     }
 
     /* Use calloc to automagically 0-terminate the copy */
-    newitem = (paralist *) pj_calloc (1, sizeof(paralist) + len);
+    newitem = (paralist *) pj_calloc (1, sizeof(paralist) + len + 1);
     if (0==newitem)
         return 0;
     memmove(newitem->param, str, len);

src/projects.h

@@ -458,7 +458,12 @@ struct PJconsts {
 struct ARG_list {
     paralist *next;
     char used;
-    char param[1];
+#ifdef __GNUC__
+    char param[0]; /* variable-length member */
+    /* Safer to use [0] for gcc. See /~https://github.com/OSGeo/proj.4/pull/1087 */
+#else
+    char param[1]; /* variable-length member */
+#endif
 };