Throw an error and hint if SYMLINK is defined on the command-line

Since SYMLINK=1 is valid/suggested for install/GNUmakefile, this is a common
mistake that we can catch here.

src/sbbs3/GNUmakefile

@@ -11,6 +11,10 @@
 # Optional build targets: dlls, utils, mono, all (default)				#
 #########################################################################
 
+ifdef SYMLINK
+$(error SYMLINK was defined and unexpected. You might mean the 'symlinks' target instead)
+endif
+
 PWD	:=	$(shell /bin/pwd)
 SRC_ROOT	?=	${PWD}/..
 include $(SRC_ROOT)/build/Common.gmake