mu-server: try avoiding xapian multi-threaded access

Try to avoid multi-threaded operations with Xapian.

This remove the thread workers during indexing, and avoids the indexing
background thread. So, mu4e has to wait once again during indexing.

We can improve upon that, but first we need to know if it avoids the
problem of issue #2756.

meson.build

@@ -133,7 +133,13 @@ add_project_arguments(['-DHAVE_CONFIG_H'], language: 'cpp')
 config_h_dep=declare_dependency(
   include_directories: include_directories(['.']))
 
-
+#
+# single-threaded Xapian access?
+#
+if get_option('xapian-single-threaded')
+  config_h_data.set('XAPIAN_SINGLE_THREADED', true)
+  message('use Xapian only in a single thread')
+endif
 #
 # d_type, d_ino are not available universally, so let's check
 # (we use them for optimizations in mu-scanner
@@ -322,6 +328,8 @@ if gmime_dep.version() == '3.2.13'
   warning('See: https://github.com/jstedfast/gmime/issues/133')
 endif
 
+
+
 # Local Variables:
 # indent-tabs-mode: nil
 # End: