Brotli: fix build.

/bin/sh's equal operator is "=" not "==".

Reported by rkd77 on GitHub (issue #3).

Signed-off-by: Piotr Sikora <piotrsikora@google.com>

auto/feature

diff --git a/auto/feature b/auto/feature
index 92294b7..eede183 100644
--- a/auto/feature
+++ b/auto/feature
@@ -25,7 +25,7 @@
     done
 fi
 
-if [ "$ngx_feature_cpp" == "yes" ]; then
+if [ "$ngx_feature_cpp" = "yes" ]; then
     NGX_AUTOTEST_EXT=cc
 else
     NGX_AUTOTEST_EXT=c