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/meson.build

@@ -0,0 +1,39 @@
+plugindir = join_paths(get_option('libdir'), 'hexchat/plugins')
+
+if host_machine.system() == 'windows'
+  if get_option('with-exec')
+    subdir('exec')
+  endif
+
+  if get_option('with-upd')
+    subdir('upd')
+  endif
+
+  if get_option('with-winamp')
+    subdir('winamp')
+  endif
+endif
+
+if get_option('with-checksum')
+  subdir('checksum')
+endif
+
+if get_option('with-fishlim')
+  subdir('fishlim')
+endif
+
+if get_option('with-lua') != 'false'
+  subdir('lua')
+endif
+
+if get_option('with-perl')
+  subdir('perl')
+endif
+
+if get_option('with-python') != 'false'
+  subdir('python')
+endif
+
+if get_option('with-sysinfo')
+  subdir('sysinfo')
+endif