lame: fix build if host has libgtk12-dev (gtk-config)

Lame contains an optional gtk12 based frame analyzer application, which
will be built if gtk12 is detected (using gtk-config). We no longer
have support for (the obsolete) gtk12 in BR, but the host might have
gtk-config, so forcible disable it.

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

package/multimedia/lame/lame.mk

@@ -8,6 +8,7 @@ LAME_VERSION = 3.98.4
 LAME_SITE = http://$(BR2_SOURCEFORGE_MIRROR).dl.sourceforge.net/sourceforge/lame
 LAME_DEPENDENCIES = host-pkg-config
 LAME_INSTALL_STAGING = YES
+LAME_CONF_ENV = GTK_CONFIG=/bin/false
 
 ifeq ($(BR2_PACKAGE_LIBSNDFILE),y)
 LAME_DEPENDENCIES += libsndfile