Build without -Werror

It makes sense for upstream CI and with carefully controlled compiler
versions, but it is too strict for downstream use.

gn.spec

@@ -1,5 +1,8 @@
 # Build HTML docs from markdown using pandoc?
 %bcond_without html_docs
+# Normally, treating warnings as errors is too strict for downstream builds.
+# It’s nice to be able to turn it on, though.
+%bcond_with werror
 
 Name:           gn
 # Upstream uses the number of commits in the git history as the version number.
@@ -123,6 +126,7 @@ AR='gcc-ar'; export AR
 # not with g++. We do get LTO on Fedora anyway, since we respect the
 # distribution’s build flags.
 %{python3} build/gen.py \
+    %{?!with_werror:--allow-warnings} \
     --no-last-commit-position \
     --no-strip \
     --no-static-libstdc++