lib/common.mk: avoid using uninitialized variables

It somehow managed to evade me that $(TMP) might be uninitialized;
definitely should be checked before stuffing into sed substitution
command.

NB: this could be done in pure make but my take was less readable.

Thanks shadowsbrother/gmail for hitting and reporting this.

lib/common.mk

@@ -7,10 +7,17 @@ BUILD_LOG = build.log
 SYMLINK = build
 
 # brevity postprocessor; not exported, for toplevel use only
-SHORTEN = $(shell \
+SHORTEN = $(shell FILTER=; \
-	echo -n "| sed"; \
 	if [ -s "$(SYMLINK)" ]; then \
-		echo -n " -e 's,$(BUILDDIR),$(SYMLINK),'"; \
+		FILTER=" -e 's,$(BUILDDIR),$(SYMLINK),'"; \
+	fi; \
+	if [ -n "$$TMP" ]; then \
+		FILTER="$$FILTER -e 's,$$TMP,\$$TMP,'"; \
+	fi; \
+	if [ -n "$$HOME" ]; then \
+		FILTER="$$FILTER -e 's,$$HOME,~,'"; \
+	fi; \
+	if [ -n "$$FILTER" ]; then \
+		echo -n "| sed $$FILTER"; \
 	fi; \
-	echo -n " -e 's,$(TMP),\$$TMP,' -e 's,$(HOME),~,'"; \
 )