Ensure that the FB frontend sets a subtarget, otherwise error out. This prevents silly mistakes like 'NETSURF_FB_FRONTEND := SDL' which had me stumped for a few minutes.

svn path=/trunk/netsurf/; revision=6571

Makefile

@@ -608,6 +608,11 @@ ifeq ($(TARGET),framebuffer)
     LDFLAGS += $(shell $(PKG_CONFIG) --libs libxml-2.0 libcurl openssl)
     SUBTARGET := -vnc
   endif
+
+  ifeq ($(SUBTARGET),)
+    $(error Unable to proceed, no FB subtarget chosen.)
+  endif
+
 endif
 
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