build: fix concurrent build failure

Reported by Dengke Du and Robert Yang.
https://lists.gnu.org/archive/html/bison-patches/2017-07/msg00000.html

* src/local.mk (src/yacc): Make sure the directory exists.

THANKS

@@ -40,6 +40,7 @@ Daniel Hagerty            hag@gnu.org
 David J. MacKenzie        djm@gnu.org
 David Kastrup             dak@gnu.org
 David Michael             fedora.dm0@gmail.com
+Dengke Du                 dengke.du@windriver.com
 Dennis Clarke             dclarke@blastwave.org
 Derek Clegg               derek@me.com
 Derek M. Jones            derek@knosof.co.uk
@@ -136,6 +137,7 @@ Rici Lake                 ricilake@gmail.com
 Rob Conde                 rob.conde@ai-solutions.com
 Rob Vermaas               rob.vermaas@gmail.com
 Robert Anisko             anisko_r@epita.fr
+Robert Yang               liezhi.yang@windriver.com
 Roland Levillain          roland@lrde.epita.fr
 Satya Kiran Popuri        satyakiran@gmail.com
 Sebastian Setzer          sebastian.setzer.ext@siemens.com