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/python/python.c

@@ -69,6 +69,8 @@
 #include "hexchat-plugin.h"
 #undef _POSIX_C_SOURCE	/* Avoid warnings from /usr/include/features.h */
 #undef _XOPEN_SOURCE
+#undef HAVE_MEMRCHR /* Avoid redefinition in Python.h */
+#undef HAVE_STRINGS_H
 #include <Python.h>
 #include <structmember.h>
 #include <pythread.h>