bison: fix m4 dependency

Bison needs m4 (at runtime). In addition, the bison 2.5 configure script
checks for m4 4.1.6 or newer, and bails out otherwise, but ends up looking
at whatever the build host has rather than the cross version, which might
not be the correct version (or even available).

Fix it by short circuiting the check.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/bison/Config.in

@@ -1,6 +1,7 @@
 config BR2_PACKAGE_BISON
 	bool "bison"
 	depends on BR2_USE_WCHAR
+	select BR2_PACKAGE_M4
 	help
 	  General-purpose parser generator that converts a
 	  grammar description for an LALR context-free grammar into a C

package/bison/bison.mk

@@ -7,11 +7,16 @@
 BISON_VERSION = 2.5
 BISON_SITE = $(BR2_GNU_MIRROR)/bison
 
+BISON_CONF_ENV += ac_cv_path_M4=/usr/bin/m4
+BISON_DEPENDENCIES += m4
+
 define BISON_DISABLE_EXAMPLES
 	echo 'all install:' > $(@D)/examples/Makefile
 endef
 
 BISON_POST_CONFIGURE_HOOKS += BISON_DISABLE_EXAMPLES
 
+HOST_BISON_DEPENDENCIES += host-m4
+
 $(eval $(call AUTOTARGETS))
 $(eval $(call AUTOTARGETS,host))