Protect included headers with the proper autoconf macros

[Mike McCormack]

>> -#ifdef HAVE_LIBXML2
>> -
>> +#ifdef HAVE_LIBXML_PARSER_H
>> #include <libxml/parser.h>
>> +#endif
>> +
>> +#ifdef HAVE_LIBXML2

> This patch causes the compile error for me. I have libxml2 headers
> but I don't have a library. XML2INCL is defined only if the configure
> script found library so gcc cannot find my headers at compile time.
> If you want to change it, you'll have to change configure.ac as well,
> but why do you want to include libxml if it won't be used anyway?

Probably better to do:

#ifdef HAVE_LIBXML2

#ifdef HAVE_LIBXML_PARSER_H
#include <libxml/parser.h>
#endif

...

#endif

That way we have the right include guard and we'll only compile if we 
have the library too.

Mike