Fix rebuilding docs, by hiding __attribute__((...)) behind a macro.
When enabled via `./configure --enable-rebuild-docs`,
`make -C doc libxml2-api.xml` will invoke apibuild.py
to rebuild libxml2-api.xml from the sources.
But the code added in
9fa3200cb366c726f7c8ef234282603bb9e8816d made it error out with
```
Parsing ../parser.c
Parse Error: parsing type : expecting a name
('Got token ', ('sep', '('))
('Last token: ', ('sep', '('))
('Token queue: ', [('name', 'destructor'), ('sep', ')'), ('sep', ')')])
('Line 14689 end: ', '')
```
3 files changed
