build: fix compiler warning

guile: some clang warning

and ensure we get the right Xapian in all places, and fileno is defined.

lib/index/meson.build

@@ -21,8 +21,9 @@ index_srcs=[
 	'mu-scanner.cc'
 ]
 
+xapian_incs = xapian_dep.get_pkgconfig_variable('includedir')
 lib_mu_index_inc_dep = declare_dependency(
-  include_directories: include_directories(['.', '..']))
+  include_directories: include_directories(['.', '..', xapian_incs]))
 lib_mu_index=static_library('mu-index', [index_srcs],
 			    dependencies: [
 			      config_h_dep,