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/fishlim/meson.build

@@ -0,0 +1,18 @@
+if not libssl_dep.found()
+  error('fish plugin requires openssl')
+endif
+
+fishlim_sources = [
+  'dh1080.c',
+  'fish.c',
+  'irc.c',
+  'keystore.c',
+  'plugin_hexchat.c'
+]
+
+shared_module('fishlim', fishlim_sources,
+  dependencies: [libgio_dep, hexchat_plugin_dep, libssl_dep],
+  install: true,
+  install_dir: plugindir,
+  name_prefix: '',
+)