flex: needs M4 at runtime

For proper runtime execution, flex requires m4 to be
installed. Passing a M4 variable at configure time is needed,
otherwise flex on the target will try to use a 'm4' binary with a
build machine path.

Fixes bug #4988.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/flex/Config.in

@@ -12,5 +12,7 @@ config BR2_PACKAGE_FLEX_BINARY
 	# needs fork()
 	depends on BR2_USE_MMU
 	depends on BR2_PACKAGE_FLEX
+	# runtime dependency
+	select BR2_PACKAGE_M4
 	help
 	  Install the flex binary tool in the target filesystem.

package/flex/flex.mk

@@ -11,6 +11,7 @@ FLEX_LICENSE = FLEX
 FLEX_LICENSE_FILES = COPYING
 FLEX_DEPENDENCIES = \
 	$(if $(BR2_PACKAGE_GETTEXT_IF_LOCALE),gettext) host-m4
+FLEX_CONF_ENV = M4=/usr/bin/m4
 # we don't have a host-gettext/libintl
 HOST_FLEX_DEPENDENCIES = host-m4