spice: convert to meson

Faster compilation.

Removed autotools patches.

Backported meson patch to fix compilation and pkgconfig file.

Before:

time make package/spice/compile -j 12
Executed in   62.85 secs   fish           external
   usr time   56.45 secs  276.00 micros   56.45 secs
   sys time    8.06 secs   46.00 micros    8.06 secs

Before + PKG_BUILD_PARALLEL:

Executed in   45.40 secs   fish           external
   usr time   63.08 secs  253.00 micros   63.08 secs
   sys time    8.57 secs   44.00 micros    8.57 secs

After:

time make package/spice/compile -j 12
Executed in   16.54 secs   fish           external
   usr time   41.29 secs  266.00 micros   41.29 secs
   sys time    4.76 secs   45.00 micros    4.76 secs

Signed-off-by: Rosen Penev <rosenp@gmail.com>

libs/spice/patches/050-no-mkenums.patch

@@ -0,0 +1,29 @@
+From 60f49745eb80eb84ff293cfadf8092b66b6a088c Mon Sep 17 00:00:00 2001
+From: Rosen Penev <rosenp@gmail.com>
+Date: Fri, 9 Oct 2020 04:22:31 -0700
+Subject: [PATCH] don't regenerate enums files
+
+The release tarballs already include these files.
+
+Signed-off-by: Rosen Penev <rosenp@gmail.com>
+---
+ server/meson.build | 6 +-----
+ 1 file changed, 1 insertion(+), 5 deletions(-)
+
+diff --git a/server/meson.build b/server/meson.build
+index b2eb410..b79b8d9 100644
+--- a/server/meson.build
++++ b/server/meson.build
+@@ -36,11 +36,7 @@ spice_server_headers = [
+ 
+ install_headers(spice_server_headers, subdir : 'spice-server')
+ 
+-# generate enums
+-gnome = import('gnome')
+-spice_server_enums = gnome.mkenums_simple('spice-server-enums',
+-                                          sources : 'spice-server.h',
+-                                          symbol_prefix : 'SPICE')
++spice_server_enums = ['spice-server-enums.c', 'spice-server-enums.h']
+ 
+ spice_server_sources = [
+   spice_server_headers,