#include ordering for source files

• #include <config.h> should be first, and use <> not "" (as recommended by the autoconf manual).

• The header corresponding to the source file should be next. This means that compilation of the library ensures that each header with a corresponding source file is "self supporting" (i.e. it implicitly or explicitly includes all of the headers it requires).

• Not sure what's best for the rest - I don't yet have coherent arguments for a particular ordering.

• External xapian-core headers, alphabetically. When included from other external headers, use <> to reduce problems with finding headers in the user's source tree by mistake. In sources and internal headers, use "" (?) - practically this makes no difference as we have -I for srcdir and builddir, but <> suggests installed header files so "" seems more natural).

• Internal headers, alphabetically (using "").

• "Safe" versions of library headers (include these first to avoid issues if other library headers include the ones we want to wrap). Use "" and order alphabetically.

• Library headers, alphabetically.

• Standard C++ headers, alphabetically. Use the modern (no .h suffix) names.