Add doxygen build target.

BUG=none
R=jvoung@chromium.org, stichnot@chromium.org

Review URL: https://codereview.chromium.org/1207823002.
diff --git a/.gitignore b/.gitignore
index 062ad19..8cda9be 100644
--- a/.gitignore
+++ b/.gitignore
@@ -10,4 +10,5 @@
 # Ignore specific patterns at the top-level directory
 /pnacl-sz
 /build/
+/docs/
 /crosstest/Output/