Build: Make function constructor/destructor attributes mandatory.

This change causes the configure script to fail if the C compiler does
not support both function attributes constructor and destructor.  These
were already required as modules cannot function without these attributes
and Asterisk requires modules.

This also has AST_GCC_ATTRIBUTE set a variable
ax_cv_have_func_attribute_$1.  This is the same variable name used by
autoconf-archive's AX_GCC_FUNC_ATTRIBUTE, used for the same purpose.

Change-Id: Id68e8a1447f2a6d707c54b56350e7bfdb33fb663