diff --git a/tools/build/mk/Makefile.boot b/tools/build/mk/Makefile.boot
index 1d121e916a71..b83f417b524f 100644
--- a/tools/build/mk/Makefile.boot
+++ b/tools/build/mk/Makefile.boot
@@ -96,3 +96,6 @@ UPDATE_DEPENDFILE= no
     Error was caused by Makefile in ${.CURDIR}
 .endif
 .endif
+
+# GCC doesn't allow silencing warn_unused_result calls with (void) casts.
+CFLAGS.gcc+=-Wno-unused-result