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/pkgconfig/Makefile.am

@@ -1,4 +0,0 @@
-pkgcfgdir = $(pkgconfigdir)
-pkgcfg_DATA = hexchat-plugin.pc
-
-EXTRA_DIST = hexchat-plugin.pc.in

data/pkgconfig/meson.build

@@ -0,0 +1,14 @@
+pkg_conf = configuration_data()
+prefix = get_option('prefix')
+pkg_conf.set('prefix', prefix)
+pkg_conf.set('VERSION', meson.project_version())
+pkg_conf.set('hexchatlibdir', join_paths(prefix, plugindir))
+pkg_conf.set('includedir', join_paths(prefix, get_option('includedir')))
+
+configure_file(
+  input: 'hexchat-plugin.pc.in',
+  output: 'hexchat-plugin.pc',
+  configuration: pkg_conf,
+  install: true,
+  install_dir: join_paths(get_option('libdir'), 'pkgconfig'),
+)