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            [email protected]
 David J. MacKenzie        [email protected]
 David Kastrup             [email protected]
 David Michael             [email protected]
+Dengke Du                 [email protected]
 Dennis Clarke             [email protected]
 Derek Clegg               [email protected]
 Derek M. Jones            [email protected]
@@ -136,6 +137,7 @@ Rici Lake                 [email protected]
 Rob Conde                 [email protected]
 Rob Vermaas               [email protected]
 Robert Anisko             [email protected]
+Robert Yang               [email protected]
 Roland Levillain          [email protected]
 Satya Kiran Popuri        [email protected]
 Sebastian Setzer          [email protected]

src/local.mk

@@ -119,6 +119,7 @@ MOSTLYCLEANFILES += src/yacc
 
 src/yacc:
 	$(AM_V_GEN)rm -f $@ $@.tmp
+	$(AM_V_at)$(MKDIR_P) src
 	$(AM_V_at)echo '#! /bin/sh' >$@.tmp
 	$(AM_V_at)echo "exec '$(bindir)/bison' -y "'"$$@"' >>$@.tmp
 	$(AM_V_at)chmod a+x $@.tmp