package/dash: don't build in paralle

dash has races in its Makefile, but upstream is not too keen in fixing
those, and just suggests that dash not be built in parallel:

    https://www.mail-archive.com/dash@vger.kernel.org/msg01675.html

Fixes:
    http://autobuild.buildroot.org/results/793/7934c815a3009af688c2f1183e67dfe542c9a009/
    http://autobuild.buildroot.org/results/fc4/fc4e4ab47455ac47dd4a3a60083cec2848e74dbb/
    http://autobuild.buildroot.org/results/6bf/6bfdd44ea5d55a6ca3ef92254eab18c7c0416b7b/
    [...]

Reported-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/dash/dash.mk

@@ -9,6 +9,9 @@ DASH_SITE = http://gondor.apana.org.au/~herbert/dash/files
 DASH_LICENSE = BSD-3-Clause, GPL-2.0+ (mksignames.c)
 DASH_LICENSE_FILES = COPYING
 
+# dash does not build in parallel
+DASH_MAKE = $(MAKE1)
+
 ifeq ($(BR2_PACKAGE_LIBEDIT),y)
 DASH_DEPENDENCIES += libedit host-pkgconf
 DASH_CONF_OPTS += --with-libedit