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

plugins/exec/meson.build

@@ -0,0 +1,5 @@
+shared_module('exec', 'exec.c',
+  dependencies: hexchat_plugin_dep,
+  install: true,
+  install_dir: plugindir
+)