gob2: workaround configure misdetecting library needed for flex

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

package/gob2/gob2.mk

@@ -6,6 +6,7 @@
 GOB2_VERSION = 2.0.15
 GOB2_SOURCE = gob2-$(GOB2_VERSION).tar.gz
 GOB2_SITE = http://ftp.5z.com/pub/gob/
+IPERF_CONF_ENV = ac_cv_lib_lex=-lfl
 
 GOB2_DEPENDENCIES = libglib2 flex bison host-pkg-config host-flex