This patch is taken from debian's glibc package (generate-supported.mk).
It install the localedata/SUPPORTED file of the glibc. This file lists
all the supported locales of the glibc.

diff --git a/localedata/Makefile b/localedata/Makefile
index 65079f9eb8..14818f84e0 100644
--- a/localedata/Makefile
+++ b/localedata/Makefile
@@ -169,7 +169,8 @@ endif
 # Files to install.
 install-others := $(addprefix $(inst_i18ndir)/, \
 			      $(addsuffix .gz, $(charmaps)) \
-			      $(locales))
+			      $(locales)) \
+                   $(inst_i18ndir)/SUPPORTED
 
 tests: $(objdir)/iconvdata/gconv-modules
 
@@ -380,6 +381,14 @@ endif
 
 include SUPPORTED
 
+$(inst_i18ndir)/SUPPORTED: SUPPORTED $(+force)
+	for locale in $(SUPPORTED-LOCALES); do \
+		[ $$locale = true ] && continue; \
+		echo $$locale | sed 's,/, ,' >> LOCALES; \
+	done
+	$(make-target-directory)
+	$(INSTALL_DATA) LOCALES $@
+
 INSTALL-SUPPORTED-LOCALE-ARCHIVE=$(addprefix install-archive-, $(SUPPORTED-LOCALES))
 INSTALL-SUPPORTED-LOCALE-FILES=$(addprefix install-files-, $(SUPPORTED-LOCALES))
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<div class='path'>path: <a href='/guix/log/?id=0e26c3cfcf0e7fe74f942c5278a8fe6110a7ab38'>root</a>/<a href='/guix/log/gnu?id=0e26c3cfcf0e7fe74f942c5278a8fe6110a7ab38'>gnu</a>/<a href='/guix/log/gnu/services?id=0e26c3cfcf0e7fe74f942c5278a8fe6110a7ab38'>services</a>/<a href='/guix/log/gnu/services/vpn.scm?id=0e26c3cfcf0e7fe74f942c5278a8fe6110a7ab38'>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=0e26c3cfcf0e7fe74f942c5278a8fe6110a7ab38&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></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></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></td><td>Julien Lepiller</td></tr>
<tr><td><span title='2017-10-10 22:06:08 +0200'>2017-10-10</span></td><td><a href='/guix/commit/gnu/services/vpn.scm?id=4b8b4418e609b5e0bfb6efbc11ac28deaa437e80'>services: vpn: Fix default tls-auth configuration.</a><span class='msg-avail'>...</span></td><td>Julien Lepiller</td></tr>
<tr><td><span title='2017-03-31 20:55:05 -0500'>2017-03-31</span></td><td><a href='/guix/commit/gnu/services/vpn.scm?id=9fc221b58cecd73e22cac01cb0918a774a4a7b2f'>doc: Fix typos.</a><span class='msg-avail'>...</span></td><td>Eric Bavier</td></tr>
<tr><td><span title='2017-03-24 16:09:18 +0100'>2017-03-24</span></td><td><a href='/guix/commit/gnu/services/vpn.scm?id=85ac401acf2e33e4cb6d1510ac24867dc516e656'>services: openvpn: Fix a typo which was corrected in generated doc.</a><span class='msg-avail'>...</span></td><td>Mathieu Othacehe</td></tr>
<tr><td><span title='2017-01-25 14:05:37 +0100'>2017-01-25</span></td><td><a href='/guix/commit/gnu/services/vpn.scm?id=e57bd0bed86caac304e5f37c5653e6b13858c7c5'>services: Fix 'mkdir-p' in activation scripts.</a><span class='msg-avail'>...</span></td><td>Clément Lassieur</td></tr>
<tr><td><span title='2017-01-20 21:39:17 +0100'>2017-01-20</span></td><td><a href='/guix/commit/gnu/services/vpn.scm?id=be051880c435d19e7788d5df56c062f7797e0a49'>services: Reindent vpn.scm.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2017-01-15 12:24:52 +0100'>2017-01-15</span></td><td><a href='/guix/commit/gnu/services/vpn.scm?id=2be1b4712d362fa9face12a731e75038ae9d59ba'>gnu: Add openvpn service.</a><span class='msg-avail'>...</span></td><td>Julien Lepiller</td></tr>