Allow warnings in the build

Turning all warnings into errors is too strict for downstream packaging.

gn.spec

@@ -116,10 +116,13 @@ cp -vp misc/vim/README.md README-vim.md
 %build
 AR='gcc-ar'; export AR
 %set_build_flags
+# Treating warnings as errors is too strict for downstream builds.
+#
 # Both --use-icf and --use-lto add compiler flags that only work with clang++,
 # not with g++. We do get LTO on Fedora anyway, since we respect the
 # distribution’s build flags.
 %{python3} build/gen.py \
+    --allow-warnings \
     --no-last-commit-position \
     --no-strip \
     --no-static-libstdc++