New macros for controlling use of weak linkage

- `UNITY_WEAK_ATTRIBUTE`, if defined, is placed before declarations of weakly
   linked symbols. If not manually defined, it will be automatically set to
   `__attribute__((weak))` on GCC and Clang, except for Clang for Win32.
 - `UNITY_WEAK_PRAGMA`, if defined, will cause preprocessor to emit
   `#pragma weak setUp`, etc. Ignored if `UNITY_WEAK_ATTRIBUTE` is defined.
 - `UNITY_NO_WEAK` undefines both of the above resulting in no weakly
   linked symbols.

Work around for ThrowTheSwitch/Unity#93

src/unity.c

@@ -1101,12 +1101,17 @@ void UnityIgnore(const char* msg, const UNITY_LINE_TYPE line)
 }
 
 //-----------------------------------------------
-#ifdef UNITY_SUPPORT_WEAK
-UNITY_WEAK void setUp(void) { }
-UNITY_WEAK void tearDown(void) { }
+#if defined(UNITY_WEAK_ATTRIBUTE)
+    UNITY_WEAK_ATTRIBUTE void setUp(void) { }
+    UNITY_WEAK_ATTRIBUTE void tearDown(void) { }
+#elif defined(UNITY_WEAK_PRAGMA)
+#   pragma weak setUp
+    void setUp(void);
+#   pragma weak tearDown
+    void tearDown(void);
 #else
-void setUp(void);
-void tearDown(void);
+    void setUp(void);
+    void tearDown(void);
 #endif
 //-----------------------------------------------
 void UnityDefaultTestRun(UnityTestFunction Func, const char* FuncName, const int FuncLineNum)