diff --git a/libredwg.changes b/libredwg.changes
index 4cc9134..d94ce0d 100644
--- a/libredwg.changes
+++ b/libredwg.changes
@@ -1,3 +1,8 @@
+-------------------------------------------------------------------
+Mon Aug 21 09:42:33 UTC 2023 - Jan Engelhardt <jengelh@inai.de>
+
+- Set -D_GNU_SOURCE to work around strcasestr being undeclared
+
 -------------------------------------------------------------------
 Sat Jul 15 17:21:54 UTC 2023 - Jan Engelhardt <jengelh@inai.de>
 
diff --git a/libredwg.spec b/libredwg.spec
index 2772938..3b2af14 100644
--- a/libredwg.spec
+++ b/libredwg.spec
@@ -77,6 +77,7 @@ OpenDWG libraries. DWG is the native file format of AutoCAD.
 # No management of SO version despite ABI breaking changes:
 # Force-add some symvers so RPM can produce meaningful deps.
 echo 'V_%version { global: *; };' >src/sv.sym
+export CFLAGS="%optflags -D_GNU_SOURCE=1"
 %configure --disable-static
 %make_build libredwg_la_LDFLAGS=-Wl,-version-script,sv.sym libredwg_la_LIBADD=-lm