fix gcc build, forward-declare structs in the header.

MSVC allows structs to be declared in a function signature, gcc wants
to know about them earlier.

src/alchemy.h

@@ -23,6 +23,12 @@ OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 extern "C" {
 #endif
 
+    struct potion_type;
+    struct unit;
+    struct region;
+    struct item_type;
+    struct order;
+    
   enum {
     /* Stufe 1 */
     P_FAST,

src/gmtool.h

@@ -14,10 +14,12 @@
 #ifdef __cplusplus
 extern "C" {
 #endif
+
     struct lua_State;
     struct selection;
     struct state;
     struct region;
+    struct terrain_type;
 
   int gmmain(int argc, char *argv[]);
   int curses_readline(struct lua_State *L, char *buffer, size_t size,

src/kernel/connection.h

@@ -25,6 +25,13 @@ OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 extern "C" {
 #endif
 
+    struct attrib;
+    struct attrib_type;
+    struct faction;
+    struct region;
+    struct storage;
+    struct unit;
+    
   extern int nextborder;
 
   typedef struct connection {

src/kernel/equipment.h

@@ -25,6 +25,8 @@ OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 extern "C" {
 #endif
     struct spell;
+    struct item;
+    struct unit;
 
     typedef struct itemdata {
         const struct item_type *itype;

src/kernel/item.h

@@ -33,6 +33,8 @@ extern "C" {
     struct resource_type;
     struct locale;
     struct troop;
+    struct item;
+    struct order;
 
   typedef struct item {
     struct item *next;

src/kernel/plane.h

@@ -22,6 +22,10 @@ OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 extern "C" {
 #endif
 
+    struct region;
+    struct plane;
+    struct storage;
+
 #define PFL_NOCOORDS        1   /* not in use */
 #define PFL_NORECRUITS      2
 #define PFL_NOALLIANCES     4

src/kernel/pool.h

@@ -22,6 +22,10 @@ OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 extern "C" {
 #endif
 
+    struct unit;
+    struct resource_type;
+    struct region;
+
 /* bitfield values for get/use/change operations */
 #define GET_SLACK      0x01
 #define GET_RESERVE    0x02

src/kernel/save.h

@@ -22,6 +22,13 @@ OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 extern "C" {
 #endif
 
+    struct attrib;
+    struct item;
+    struct storage;
+    struct spell;
+    struct spellbook;
+    struct unit;
+    
   typedef struct gamedata {
     struct storage *store;
     int version;

src/kernel/spell.h

@@ -23,6 +23,13 @@ extern "C" {
 #endif
 
     struct castorder;
+    struct unit;
+    struct region;
+    struct spell;
+    struct spell_component;
+    struct quicklist;
+    struct attrib_type;
+
   typedef int (*spell_f)(struct castorder * co);
   typedef void(*fumble_f)(const struct castorder * co);
 

src/laws.h

@@ -22,7 +22,14 @@ OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 extern "C" {
 #endif
 
-  extern int writepasswd(void);
+    struct unit;
+    struct region;
+    struct building;
+    struct faction;
+    struct order;
+    struct attrib_type;
+
+    int writepasswd(void);
     void demographics(void);
     void update_guards(void);
     void update_subscriptions(void);
@@ -32,7 +39,7 @@ extern "C" {
     bool renamed_building(const struct building * b);
     int rename_building(struct unit * u, struct order * ord, struct building * b, const char *name);
     void get_food(struct region * r);
-  extern int can_contact(const struct region *r, const struct unit *u, const struct unit *u2);
+    int can_contact(const struct region *r, const struct unit *u, const struct unit *u2);
 
 /* eressea-specific. put somewhere else, please. */
   void processorders(void);

src/move.h

@@ -26,6 +26,8 @@ extern "C" {
 #endif
 
     struct unit;
+    struct region;
+    struct region_list;
     struct ship;
     struct building_type;
 

src/spells/shipcurse.h

@@ -22,10 +22,14 @@ extern "C" {
 
     struct locale;
     struct message;
-  extern struct message *cinfo_ship(const void *obj, objtype_t typ,
+    struct ship;
+    struct unit;
+    struct curse;
+
+    struct message *cinfo_ship(const void *obj, objtype_t typ,
                                const struct curse *c, int self);
-  extern void register_shipcurse(void);
-  extern struct curse *shipcurse_flyingship(struct ship *sh, struct unit *mage,
+    void register_shipcurse(void);
+    struct curse *shipcurse_flyingship(struct ship *sh, struct unit *mage,
                                        float power, int duration);
     int levitate_ship(struct ship *sh, struct unit *mage, float power,
                       int duration);