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
--- a/localedata/Makefile
+++ b/localedata/Makefile
@@ -176,7 +176,8 @@
 else
 install-others := $(addprefix $(inst_i18ndir)/, \
 			      $(addsuffix .gz, $(charmaps)) \
-			      $(locales))
+			      $(locales)) \
+                   $(inst_i18ndir)/SUPPORTED
 endif
 
 tests: $(objdir)/iconvdata/gconv-modules
@@ -401,6 +402,14 @@
 
 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=0b666b56247724d6beff696ee342e8bf00b76cb8'>root</a>/<a href='/guix/log/tests?id=0b666b56247724d6beff696ee342e8bf00b76cb8'>tests</a>/<a href='/guix/log/tests/union.scm?id=0b666b56247724d6beff696ee342e8bf00b76cb8'>union.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/tests/union.scm?id=0b666b56247724d6beff696ee342e8bf00b76cb8&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2019-06-14 21:57:39 +0200'>2019-06-14</span></td><td><a href='/guix/commit/tests/union.scm?id=03d76577b96ba81c9921eb3a297d42db8644280b'>tests: Make builds less expensive.</a><span class='msg-avail'>...<span class='msg-tooltip'>The switch to the reduced bootstrap broke build time assumptions made by
tests, notably the assumption that GNU-MAKE-BOOT0 was cheap to build.
This commit adjusts this to make these tests cheaper.

* gnu/packages/bootstrap.scm (%bootstrap-inputs-for-tests): New variable.
* guix/tests.scm (gnu-make-for-tests): New variable.
* tests/guix-environment.sh: Use GNU-MAKE-FOR-TESTS instead of
GNU-MAKE-BOOT0.  Remove test with FINDUTILS-BOOT0.
* tests/guix-package-net.sh (boot_make): Use GNU-MAKE-FOR-TESTS.
* tests/packages.scm ("GNU Make, bootstrap"): Likewise.
* tests/profiles.scm ("profile-derivation relative symlinks, two entries"):
Likewise.
* tests/union.scm (%bootstrap-inputs): Remove.
("union-build"): Use %BOOTSTRAP-INPUTS-FOR-TESTS instead of
%BOOTSTRAP-INPUTS.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-09-23 09:46:25 +0200'>2018-09-23</span></td><td><a href='/guix/commit/tests/union.scm?id=c362904485d3c8c3f155538ab2f7cd082b334801'>bootstrap: %bootstrap-inputs+toolchain: Replace %bootstrap-inputs.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/commencement.scm (%bootstrap-inputs+toolchain): : New procedure
replacing %bootstrap-inputs to prepare for Mes bootstrap.  Update users.
* tests/union.scm (%bootstrap-inputs): New variable: set to
%bootstrap-inputs+toolchain.
</span></span></td><td>Jan Nieuwenhuizen</td></tr>
<tr><td><span title='2018-09-22 22:40:12 +0200'>2018-09-22</span></td><td><a href='/guix/commit/tests/union.scm?id=a2b2070b679ff7e92d856c7d6775f5f67ce4792d'>bootstrap: %bootstrap-inputs: Wrap input lists into thunks.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/bootstrap.scm (%bootstrap-inputs): Change to procedure.  Update
users; prepares for Mes bootstrap.
* gnu/packages/commencement.scm (%boot0-inputs, %boot1-inputs, %boot2-inputs,
%boot3-inputs, %boot4-inputs, %boot5-inputs, %boot-6-inputs): Change to
procedure.  Update users.
* tests/builders.scm (%bootstrap-inputs, %bootstrap-search-paths): Make a
procedure, filter on package?.  Update users.
</span></span></td><td>Jan Nieuwenhuizen</td></tr>
<tr><td><span title='2018-05-10 14:53:56 +0200'>2018-05-10</span></td><td><a href='/guix/commit/tests/union.scm?id=dac1c97d131d297134fa878ac240d9ec0127044b'>union: Add 'relative-file-name'.</a><span class='msg-avail'>...<span class='msg-tooltip'>* guix/build/union.scm (%not-slash): New variable.
(relative-file-name): New procedure.
* tests/union.scm (test-relative-file-name): New macro and tests.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-04-08 17:41:08 +0200'>2018-04-08</span></td><td><a href='/guix/commit/tests/union.scm?id=e40aa54e98aa6329e6196fd29e7e4e34ce3a063c'>union: Allow callers to choose the collision resolution policy.</a><span class='msg-avail'>...<span class='msg-tooltip'>* guix/build/union.scm (warn-about-collision): New procedure.
(union-build): Add #:resolve-collision.
[resolve-collisions]: Call it.
* tests/union.scm ("union-build collision first &amp; last"): New test.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2017-03-26 12:53:48 +0200'>2017-03-26</span></td><td><a href='/guix/commit/tests/union.scm?id=addce19e2d38a197f5ea10eefb5f3cd25c3a52e7'>union: Add create-all-directories? parameter to 'union-build'.</a><span class='msg-avail'>...<span class='msg-tooltip'>* guix/build/union.scm (union-build): Add create-all-directories? keyword
parameter.
* tests/union.scm ("union-build #:create-all-directories? #t"): New test.

Co-authored-by: Ludovic Courtès &lt;ludo@gnu.org&gt;
</span></span></td><td>Huang Ying</td></tr>