Backport change from trunk:
common/pretty.h: Fix debug logging code to compile with clang.
(fixes #657, reported by Germán M. Bravo)

git-svn-id: file:///home/xapian-svn/xapian/branches/1.2@18270 6b2f1b92-a4f3-0310-943f-de3cbe41748b
backends/brass/brass_database.cc,
backends/brass/brass_spellingwordslist.cc,
backends/chert/chert_database.cc,
backends/chert/chert_spellingwordslist.cc,matcher/mergepostlist.cc:
Fix mismatched return type in debug logging annotations so that code
compiles when configured with --enable-log.  These are already fixed
on trunk.

git-svn-id: file:///home/xapian-svn/xapian/branches/1.2@18271 6b2f1b92-a4f3-0310-943f-de3cbe41748b