diff --git a/libguestfs.spec b/libguestfs.spec
index ced7f18..9c18187 100644
--- a/libguestfs.spec
+++ b/libguestfs.spec
@@ -208,7 +208,9 @@ virtual machines.
 %patch -p1
 
 %build
-export RPM_OPT_FLAGS="$RPM_OPT_FLAGS -Wno-unused"
+# for configure macro below
+CFLAGS="$RPM_OPT_FLAGS -Wno-unused"
+CXXFLAGS="$RPM_OPT_FLAGS -Wno-unused"
 autoreconf -fi
 %configure \
 	--help || :