build: Replace Autotools with Meson

Quick rundown of benefits:

- Much faster:
  - Autotools (with autogen): 22 seconds
  - Meson: 7 seconds
  - Meson (with ccache): 2 seconds

- Simpler:
  - ~1000 lines smaller
  - Single simple language

- Potentially better Windows (Visual Studio) support

What is not done:

- Complete Windows support
- OSX support (easy)

Closes #2013
Closes #1937
Closes #1803

data/man/meson.build

@@ -0,0 +1,10 @@
+man_conf = configuration_data()
+man_conf.set('VERSION', meson.project_version())
+
+configure_file(
+  input: 'hexchat.1.in',
+  output: 'hexchat.1',
+  configuration: man_conf,
+  install: true,
+  install_dir: join_paths(get_option('mandir'), 'man1')
+)