Add support for CONFIG_NO_ROAMING to Makefile

Commit e9af53ad39a19264bf5307a1b5923afc2b2f9b06 introduced new
CONFIG_NO_ROAMING configuration parameter but unfortunately it was added
only to Android.mk. Enabling this parameter didn't have any effect when
Makefile was used to build wpa_supplicant. This commit fixes that problem
and cleans "unused variable" compiler warning.

Signed-off-by: Lauri Hintsala <lauri.hintsala@silabs.com>
diff --git a/wpa_supplicant/Makefile b/wpa_supplicant/Makefile
index 662e7f8..af2d924 100644
--- a/wpa_supplicant/Makefile
+++ b/wpa_supplicant/Makefile
@@ -317,6 +317,10 @@
 NEED_GAS=y
 endif
 
+ifdef CONFIG_NO_ROAMING
+CFLAGS += -DCONFIG_NO_ROAMING
+endif
+
 include ../src/drivers/drivers.mak
 ifdef CONFIG_AP
 OBJS_d += $(DRV_BOTH_OBJS)
diff --git a/wpa_supplicant/events.c b/wpa_supplicant/events.c
index b0671bf..f5f3032 100644
--- a/wpa_supplicant/events.c
+++ b/wpa_supplicant/events.c
@@ -1286,7 +1286,9 @@
 				       struct wpa_ssid *ssid)
 {
 	struct wpa_bss *current_bss = NULL;
+#ifndef CONFIG_NO_ROAMING
 	int min_diff;
+#endif /* CONFIG_NO_ROAMING */
 
 	if (wpa_s->reassociate)
 		return 1; /* explicit request to reassociate */