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/checksum/Makefile.am

@@ -1,8 +0,0 @@
-libdir = $(hexchatlibdir)
-
-lib_LTLIBRARIES = checksum.la
-checksum_la_SOURCES = checksum.c
-checksum_la_LDFLAGS = $(PLUGIN_LDFLAGS) -module
-checksum_la_LIBADD = $(GLIB_LIBS)
-checksum_la_CPPFLAGS = -I$(top_srcdir)/src/common
-checksum_la_CFLAGS = $(GLIB_CFLAGS)

plugins/checksum/meson.build

@@ -0,0 +1,6 @@
+shared_module('checksum', 'checksum.c',
+  dependencies: [libgio_dep, hexchat_plugin_dep],
+  install: true,
+  install_dir: plugindir,
+  name_prefix: '',
+)