try to fix warning for flexible array member

src/proj_include/proj_internal.h

@@ -527,8 +527,8 @@ struct PJconsts {
     double  datum_params[7] = {0,0,0,0,0,0,0}; /* Parameters for 3PARAM and 7PARAM */
 
     int     has_geoid_vgrids = 0;      /* used by legacy transform.cpp */
-    void*   hgrids_legacy = nullptr;   /* used by legacy transform.cpp. Is a pointer to a ListOfHGrids* */ 
-    void*   vgrids_legacy = nullptr;   /* used by legacy transform.cpp. Is a pointer to a ListOfVGrids* */ 
+    void*   hgrids_legacy = nullptr;   /* used by legacy transform.cpp. Is a pointer to a ListOfHGrids* */
+    void*   vgrids_legacy = nullptr;   /* used by legacy transform.cpp. Is a pointer to a ListOfVGrids* */
 
     double  from_greenwich = 0.0;       /* prime meridian offset (in radians) */
     double  long_wrap_center = 0.0;     /* 0.0 for -180 to 180, actually in radians*/
@@ -573,7 +573,7 @@ struct ARG_list {
     paralist *next;
     char used;
 #if (defined(__GNUC__) && __GNUC__ >= 8) || (defined(__clang__) && __clang_major__ >= 9)
-    char param[]; /* variable-length member */
+    __extension__ char param[]; /* variable-length member */
     /* Safer to use [] for gcc 8. See /~https://github.com/OSGeo/proj.4/pull/1087 */
     /* and https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86914 */
 #else