Fix build failure when using Swig 4.

Taken from upstream:
https://github.com/fifengine/fifengine/commit/d14f232f4cd9a00b05d6872957070e8c020f515d

diff --git a/CMakeLists.txt b/CMakeLists.txt
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -893,7 +893,7 @@ if(build-python)
   find_package(PythonLibs ${PYTHON_VERSION_STRING} EXACT)
   include_directories(${PYTHON_INCLUDE_PATH})
 
-  set(CMAKE_SWIG_FLAGS -modern -fastdispatch -dirvtable -nosafecstrings -noproxydel -fastproxy -fastinit -fastunpack -fastquery -modernargs -nobuildnone -keyword -w511 -w473 -w404 -w314)
+  set(CMAKE_SWIG_FLAGS -modern -fastdispatch -dirvtable -noproxydel -fastproxy -fastinit -fastunpack -fastquery -modernargs -keyword -w511 -w473 -w404 -w314)
 
   set_source_files_properties("${PROJECT_BINARY_DIR}/fife.i" PROPERTIES CPLUSPLUS ON)
   set(FIFE_SOURCES ${FIFE_CORE_SRC})
put type='submit' value='switch'/></form></td></tr>
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<div class='path'>path: <a href='/guix/log/?id=2d092a2afae60421157b486c4056f12ba6d0d20a'>root</a>/<a href='/guix/log/gnu?id=2d092a2afae60421157b486c4056f12ba6d0d20a'>gnu</a>/<a href='/guix/log/gnu/services?id=2d092a2afae60421157b486c4056f12ba6d0d20a'>services</a>/<a href='/guix/log/gnu/services/vpn.scm?id=2d092a2afae60421157b486c4056f12ba6d0d20a'>vpn.scm</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/gnu/services/vpn.scm?id=2d092a2afae60421157b486c4056f12ba6d0d20a&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2021-04-20 14:32:53 +0200'>2021-04-20</span></td><td><a href='/guix/commit/gnu/services/vpn.scm?id=3313f61e18cbb4a89ec6c980b39f8e5fcad2b890'>services: wireguard: Add keep-alive support.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/services/vpn.scm (&lt;wireguard-peer&gt;): Add 'keep-alive' field.
  (wireguard-configuration-file): Use it.
* doc/guix.texi (VPN Services): Document it.
</span></span></td><td>Guillaume Le Vaillant</td></tr>
<tr><td><span title='2021-02-17 10:32:15 +0100'>2021-02-17</span></td><td><a href='/guix/commit/gnu/services/vpn.scm?id=43b2e440c38a39eb64088bd6c08771c060aa10fc'>services: wireguard: New service.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/services/vpn.scm (wireguard-peer, wireguard-configuration): New records.
(wireguard-service-type): New variable.
* doc/guix.texi (VPN Services): Document it.
</span></span></td><td>Mathieu Othacehe</td></tr>
<tr><td><span title='2020-11-18 15:05:41 +0100'>2020-11-18</span></td><td><a href='/guix/commit/gnu/services/vpn.scm?id=5221df34149465c5bbc1a76f83cb09f8911279f5'>gnu: vpn: Make ca, key and cert optional.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/services/vpn.scm (openvpn-client-configuration)
(openvpn-server-configuration): Make ca, key an cert fields optional.
* doc/guix.texi (VPN Services): Document the change.
</span></span></td><td>Julien Lepiller</td></tr>
<tr><td><span title='2020-02-19 04:08:55 +0100'>2020-02-19</span></td><td><a href='/guix/commit/gnu/services/vpn.scm?id=c6c447701c9cfdeedf77224399faa9c07b12d045'>gnu: services: Add openvpn options.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/services/vpn.scm (openvpn-client-configuration)
(openvpn-server-configuration): Add fast-io? and auth-user-pass options.
</span></span></td><td>Julien Lepiller</td></tr>
<tr><td><span title='2020-02-19 04:08:40 +0100'>2020-02-19</span></td><td><a href='/guix/commit/gnu/services/vpn.scm?id=0372dd1a1e51e39382ee5d1aa968589b40506b8f'>gnu: services: Fix openvpn boolean fields.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/services/vpn.scm (serialize-boolean): Do not print #t to the file.
</span></span></td><td>Julien Lepiller</td></tr>