Add support for the clang compiler; update RAII_VAR to use BlocksRuntime

RAII_VAR, which is used extensively in Asterisk to manage reference counted
resources, uses a GCC extension to automatically invoke a cleanup function
when a variable loses scope. While this functionality is incredibly useful
and has prevented a large number of memory leaks, it also prevents Asterisk
from being compiled with clang.

This patch updates the RAII_VAR macro such that it can be compiled with clang.
It makes use of the BlocksRuntime, which allows for a closure to be created
that performs the actual cleanup.

Note that this does not attempt to address the numerous warnings that the clang
compiler catches in Asterisk.

Much thanks for this patch goes to:
* The folks on StackOverflow who asked this question and Leushenko for
  providing the answer that formed the basis of this code:
  http://stackoverflow.com/questions/24959440/rewrite-gcc-cleanup-macro-with-nested-function-for-clang
* Diederik de Groot, who has been extremely patient in working on getting this
  patch into Asterisk.

Review: https://reviewboard.asterisk.org/r/4370/

ASTERISK-24133
ASTERISK-23666
ASTERISK-20399
ASTERISK-20850 #close
Reported by: Diederik de Groot
patches:
  RAII_CLANG.patch uploaded by Diederik de Groot (License 6600)
........

Merged revisions 432807 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/13@432808 65c4cc65-6c06-0410-ace0-fbb531ad65f3

include/asterisk/utils.h

@@ -1005,12 +1005,33 @@ char *ast_utils_which(const char *binary, char *fullpath, size_t fullpath_size);
  * }
  * \endcode
  */
-#define RAII_VAR(vartype, varname, initval, dtor) \
-    /* Prototype needed due to http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36774 */ \
-    auto void _dtor_ ## varname (vartype * v); \
-    void _dtor_ ## varname (vartype * v) { dtor(*v); } \
+
+#if defined(__clang__)
+
+#if defined(__has_feature) && __has_feature(blocks)
+typedef void (^_raii_cleanup_block_t)(void);
+static inline void _raii_cleanup_block(_raii_cleanup_block_t *b) { (*b)(); }
+
+#define RAII_VAR(vartype, varname, initval, dtor)                                                                \
+    _raii_cleanup_block_t _raii_cleanup_ ## varname __attribute__((cleanup(_raii_cleanup_block),unused)) = NULL; \
+    vartype varname = initval;                                                                                   \
+    _raii_cleanup_ ## varname = ^{ dtor(varname); }
+
+#else
+	#error "CLANG must support the 'blocks' feature to compile Asterisk."
+#endif /* #if defined(__has_feature) && __has_feature(blocks) */
+
+#elif defined(__GNUC__)
+
+#define RAII_VAR(vartype, varname, initval, dtor)                              \
+    auto void _dtor_ ## varname (vartype * v);                                 \
+    void _dtor_ ## varname (vartype * v) { dtor(*v); }                         \
     vartype varname __attribute__((cleanup(_dtor_ ## varname))) = (initval)
 
+#else
+    #error "Cannot compile Asterisk: unknown and unsupported compiler."
+#endif /* #if __GNUC__ */
+
 /*!
  * \brief Asterisk wrapper around crypt(3).
  *