weakref attribute detection broken with gcc 4.6 and higher

GCC 4.7 Manual:
http://gcc.gnu.org/onlinedocs/gcc-4.7.4/gcc/Function-Attributes.html

weakref ("target")

A weak reference is an alias that does not by itself require a definition
to be given for the target symbol.

ASTERISK-22559 #close
Reported by: Ibercom

Change-Id: I36a136cae947b65187a697533416f9ff9a0b8cdf