Account for it possibly being missing.

.gitignore

@@ -38,6 +38,7 @@ deps/c-ares/stamp-h1
 deps/c-ares/stamp-h2
 deps/c-ares/ares_build.h.orig
 deps/c-ares/ares_config.h
+deps/c-ares/ares_build.h
 deps/c-ares/.libs
 deps/c-ares/*.o
 deps/c-ares/*.lo

_setupares.py

@@ -44,7 +44,7 @@ ares_configure_command = ' '.join([
     " && sh ./configure --disable-dependency-tracking " + _m32 + "CONFIG_COMMANDS= ",
     " && cp ares_config.h ares_build.h \"$OLDPWD\" ",
     " && cat ares_build.h ",
-    " && mv ares_build.h.orig ares_build.h)",
+    " && if [ -r ares_build.h.orig ]; then mv ares_build.h.orig ares_build.h; fi)",
     "> configure-output.txt"])