Refactor weak symbol usage.

The Linux run-time linker doesn't allow weak references in one library
to be overridden by ones in another.  To work around this, we now
declare the runtime functions as weak and perform dynamic checks on
whether the symbols have been resolved and call the fallbacks as local
static functions if they are not present.

Source/NSObject.m

@@ -446,8 +446,24 @@ struct obj_layout {
 };
 typedef	struct obj_layout *obj;
 
+/*
+ * These symbols are provided by newer versions of the GNUstep Objective-C
+ * runtime.  When linked against an older version, we will use our internal
+ * versions.
+ */
 __attribute__((weak))
-BOOL objc_release_fast_no_destroy_np(id anObject)
+BOOL objc_release_fast_no_destroy_np(id anObject);
+
+__attribute__((weak))
+void objc_release_fast_np(id anObject);
+
+__attribute__((weak))
+size_t object_getRetainCount_np(id anObject);
+
+__attribute__((weak))
+id objc_retain_fast_np(id anObject);
+
+static BOOL objc_release_fast_no_destroy_internal(id anObject)
 {
   if (double_release_check_enabled)
     {
@@ -506,15 +522,38 @@ BOOL objc_release_fast_no_destroy_np(id anObject)
   return NO;
 }
 
-__attribute__((weak))
-void objc_release_fast_np(id anObject)
+static BOOL release_fast_no_destroy(id anObject)
 {
-  if (objc_release_fast_no_destroy_np(anObject))
+  if (objc_release_fast_no_destroy_np)
+    {
+      return objc_release_fast_no_destroy_np(anObject);
+    }
+  else
+    {
+      return objc_release_fast_no_destroy_internal(anObject);
+    }
+}
+
+static void objc_release_fast_np_internal(id anObject)
+{
+  if (release_fast_no_destroy(anObject))
     {
       [anObject dealloc];
     }
 }
 
+static void release_fast(id anObject)
+{
+  if (objc_release_fast_np)
+    {
+      objc_release_fast_np(anObject);
+    }
+  else
+    {
+      objc_release_fast_np_internal(anObject);
+    }
+}
+
 /**
  * Examines the extra reference count for the object and, if non-zero
  * decrements it, otherwise leaves it unchanged.<br />
@@ -524,15 +563,26 @@ void objc_release_fast_np(id anObject)
 inline BOOL
 NSDecrementExtraRefCountWasZero(id anObject)
 {
-  return objc_release_fast_no_destroy_np(anObject);
+  return release_fast_no_destroy(anObject);
 }
 
-__attribute__((weak))
-size_t object_getRetainCount_np(id anObject)
+size_t object_getRetainCount_np_internal(id anObject)
 {
   return ((obj)anObject)[-1].retained + 1;
 }
 
+size_t getRetainCount(id anObject)
+{
+  if (object_getRetainCount_np)
+    {
+      return object_getRetainCount_np(anObject);
+    }
+  else
+    {
+      return object_getRetainCount_np_internal(anObject);
+    }
+}
+
 /**
  * Return the extra reference count of anObject (a value in the range
  * from 0 to the maximum unsigned integer value minus one).<br />
@@ -541,7 +591,7 @@ size_t object_getRetainCount_np(id anObject)
 inline NSUInteger
 NSExtraRefCount(id anObject)
 {
-  return object_getRetainCount_np(anObject) - 1;
+  return getRetainCount(anObject) - 1;
 }
 
 /**
@@ -550,8 +600,7 @@ NSExtraRefCount(id anObject)
  * would be incremented to too large a value.<br />
  * This is used by the [NSObject-retain] method.
  */
-__attribute__((weak))
-id objc_retain_fast_np(id anObject)
+static id objc_retain_fast_np_internal(id anObject)
 {
   BOOL  tooFar = NO;
 
@@ -617,6 +666,18 @@ id objc_retain_fast_np(id anObject)
   return anObject;
 }
 
+static id retain_fast(id anObject)
+{
+  if (objc_retain_fast_np)
+    {
+      return objc_retain_fast_np(anObject);
+    }
+  else
+    {
+      return objc_retain_fast_np_internal(anObject);
+    }
+}
+
 /**
  * Increments the extra reference count for anObject.<br />
  * The GNUstep version raises an exception if the reference count
@@ -626,7 +687,7 @@ id objc_retain_fast_np(id anObject)
 inline void
 NSIncrementExtraRefCount(id anObject)
 {
-   objc_retain_fast_np(anObject);
+   retain_fast(anObject);
 }
 
 #ifndef	NDEBUG
@@ -1967,7 +2028,7 @@ static id gs_weak_load(id obj)
  */
 - (oneway void) release
 {
-  objc_release_fast_np(self);
+  release_fast(self);
 }
 
 /**
@@ -2026,7 +2087,7 @@ static id gs_weak_load(id obj)
  */
 - (id) retain
 {
-  return objc_retain_fast_np(self);
+  return retain_fast(self);
 }
 
 /**
@@ -2050,7 +2111,7 @@ static id gs_weak_load(id obj)
  */
 - (NSUInteger) retainCount
 {
-  return object_getRetainCount_np(self);
+  return getRetainCount(self);
 }
 
 /**