bootstrap: Fix exit code when autoreconf fails (#962)

Fixes:

./bootstrap: line 37: exit: $: numeric argument required
diff --git a/bootstrap b/bootstrap
index 751cc7c..6312953 100755
--- a/bootstrap
+++ b/bootstrap
@@ -32,4 +32,4 @@
 sed -i.bak "$SED_ERE" "s/[0-9]+:[0-9]+:[0-9]+/$BROTLI_ABI_INFO/" Makefile.am
 sed -i.bak "$SED_ERE" "s/\[[0-9]+\.[0-9]+\.[0-9]+\]/[$BROTLI_VERSION]/" configure.ac
 
-autoreconf --install --force --symlink || exit $
+autoreconf --install --force --symlink || exit $?