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/perl/meson.build

@@ -0,0 +1,73 @@
+generate_perl_header = find_program('generate_header.py')
+
+hexchat_perl_module = custom_target('hexchat-perl-header',
+  input: [
+    'lib/HexChat.pm',
+    'lib/Xchat.pm',
+    'lib/HexChat/Embed.pm',
+    'lib/HexChat/List/Network.pm',
+    'lib/HexChat/List/Network/Entry.pm',
+    'lib/HexChat/List/Network/AutoJoin.pm',
+  ],
+  output: 'hexchat.pm.h',
+  command: [generate_perl_header, '@OUTPUT@', '@INPUT@']
+)
+
+irc_perl_module = custom_target('irc-perl-header',
+  input: 'lib/IRC.pm',
+  output: 'irc.pm.h',
+  command: [generate_perl_header, '@OUTPUT@', '@INPUT@']
+)
+
+perl = find_program('perl')
+
+ret = run_command([perl, '-MExtUtils::Embed', '-e', 'ccopts'])
+if ret.returncode() != 0
+  error('perl: Failed to get cflags')
+endif
+perl_cflags = []
+foreach flag : ret.stdout().strip().split(' ')
+  if flag.startswith('-I') or flag.startswith('-D')
+    perl_cflags += flag
+  endif
+endforeach
+
+ret = run_command([perl, '-MExtUtils::Embed', '-e', 'ldopts'])
+if ret.returncode() != 0
+  error('perl: Failed to get ldflags')
+endif
+perl_ldflags = []
+foreach flag : ret.stdout().strip().split(' ')
+  if flag.startswith('-L') or flag.startswith('-l')
+    perl_ldflags += flag
+  endif
+endforeach
+
+perl_cflags += [
+  # Perl has its own 'config.h' that we must override
+  # TODO: Just rename ours to something more unique.
+  '-include', meson.build_root() + '/config.h'
+]
+
+if not cc.links('''
+#define PERL_NO_INLINE_FUNCTIONS
+#include <EXTERN.h>
+#include <perl.h>
+
+int main(void) {
+  PerlInterpreter *my_perl = perl_alloc();
+  return 0;
+}
+''', args: perl_cflags + perl_ldflags)
+  error('found perl not suitable for plugin')
+endif
+
+shared_module('perl',
+  sources: ['perl.c', hexchat_perl_module, irc_perl_module],
+  dependencies: [libgio_dep, hexchat_plugin_dep],
+  c_args: perl_cflags,
+  link_args: perl_ldflags,
+  install: true,
+  install_dir: plugindir,
+  name_prefix: '',
+)