meson: bump minimum version

`doc/meson.build` used currently-deprecated functions. Replace those
with the proper alternatives, which were introduced in a newer meson
version than our requirement. As a consequence, bump minimal version.

doc/meson.build

@@ -6,7 +6,7 @@
 
 scdoc = dependency('scdoc', native: true, required: false)
 if scdoc.found()
-  scdoc_prog = find_program(scdoc.get_pkgconfig_variable('scdoc'), native: true)
+  scdoc_prog = find_program(scdoc.get_variable('scdoc'), native: true)
 
   foreach section: [5, 8]
     name = 'eg25-manager'
@@ -25,7 +25,7 @@ if scdoc.found()
       out,
       output: out,
       input: preprocessed,
-      command: ['sh', '-c', '@0@ < @INPUT@'.format(scdoc_prog.path())],
+      command: ['sh', '-c', '@0@ < @INPUT@'.format(scdoc_prog.full_path())],
       capture: true,
       install: true,
       install_dir: join_paths(get_option('mandir'), 'man@0@'.format(section)))

meson.build

@@ -10,7 +10,7 @@ project (
   'c',
   version : '0.4.2',
   license : 'GPLv3+',
-  meson_version : '>= 0.50.0',
+  meson_version : '>= 0.58.0',
   default_options :
     [
       'warning_level=1',