Rename meson_options.txt to meson.options

The former is the pre-Meson 1.1 version of defining build options, while
the latter is the newer way. Now that we've started requiring a newer
Meson version, we can move to this new form too.

meson.options

@@ -0,0 +1,78 @@
+#
+# General build options
+#
+
+option(
+  'profile',
+  type: 'combo',
+  choices: [
+    'auto',
+    'development',
+    'beta',
+    'release'
+  ],
+  description: 'Specifies the application type to be built'
+)
+option(
+  'revno',
+  type: 'string',
+  description: 'Custom revision string (default extracted from "git describe")'
+)
+option(
+  'iso_639_xml',
+  type: 'string',
+  value: '',
+  description: 'Full path to the ISO 639 XML file.'
+)
+option(
+  'iso_3166_xml',
+  type: 'string',
+  value: '',
+  description: 'Full path to the ISO 3166 XML file.'
+)
+option(
+  'valadoc',
+  type: 'feature',
+  value: 'auto',
+  description: 'Build API documentation'
+)
+
+#
+# Development options
+#
+
+option(
+  'ref_tracking',
+  type: 'feature',
+  value: 'disabled',
+  description: 'Enable Geary.BaseObject reference tracking'
+)
+
+#
+# Optional features
+#
+
+option(
+  'contractor',
+  type: 'feature',
+  value: 'disabled',
+  description: 'Install an Elementary OS a contractor file'
+)
+option(
+  'libunwind',
+  type: 'feature',
+  value: 'enabled',
+  description: 'Use libunwind for back traces in problem reports.'
+)
+option(
+  'tnef',
+  type: 'feature',
+  value: 'enabled',
+  description: 'Support Microsoft-proprietary TNEF attachments.'
+)
+option(
+  'delayed_replay_queue',
+  type: 'boolean',
+  value: false,
+  description: 'Add delay in session opening (for debug purpose only)'
+)