Disable invoking sphinx
https://bugs.gentoo.org/806613

--- a/Makefile.am
+++ b/Makefile.am
@@ -57,7 +57,6 @@
 include libmemcachedutil-1.0/include.am
 
 include clients/include.am
-include docs/include.am
 include man/include.am
 
 if BUILD_LIBMEMCACHED_PROTOCOL