build: bump meson requirement to 56.0

We need some of the new functionality in the near-future.

Fix build warnings.

guile/tests/meson.build

@@ -15,8 +15,8 @@
 ## Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
 
 
-guile_load_path=':'.join([ # meson 0.56 has project_source_root
-    join_paths(meson.source_root(), 'guile'),
+guile_load_path=':'.join([
+    join_paths(meson.project_source_root(), 'guile'),
     join_paths(meson.current_build_dir(), '..')])
 
 test('test-mu-guile',

lib/index/meson.build

@@ -21,7 +21,7 @@ index_srcs=[
 	'mu-scanner.cc'
 ]
 
-xapian_incs = xapian_dep.get_pkgconfig_variable('includedir')
+xapian_incs = xapian_dep.get_variable(pkgconfig: 'includedir')
 lib_mu_index_inc_dep = declare_dependency(
   include_directories: include_directories(['.', '..', xapian_incs]))
 lib_mu_index=static_library('mu-index', [index_srcs],

meson.build

@@ -19,7 +19,7 @@
 #
 project('mu', ['c', 'cpp'],
         version: '1.9.7',
-        meson_version: '>= 0.52.0', # debian 10
+        meson_version: '>= 0.56.0',
         license: 'GPL-3.0-or-later',
         default_options : [
           'buildtype=debugoptimized',
@@ -199,7 +199,7 @@ endif
 if not get_option('guile').disabled() and guile_dep.found()
   config_h_data.set('BUILD_GUILE', 1)
   config_h_data.set_quoted('GUILE_BINARY',
-                           guile_dep.get_pkgconfig_variable('guile'))
+                           guile_dep.get_variable(pkgconfig: 'guile'))
   #message('guile is disabled for now')
   subdir('guile')
 endif