BUILD: makefile: detect and reject recently removed linux targets

We've just removed old linux targets "linux22", "linux24", "linux24e",
"linux26" and "linux2628" and it's likely that many build scripts and
packages will still reference these. So let's have the makefile detect
these and reject with instructions instead of silently building with
incorrect options.
diff --git a/Makefile b/Makefile
index 1e1057a..d56ca43 100644
--- a/Makefile
+++ b/Makefile
@@ -741,6 +741,14 @@
 	@echo "out of it. Please check the Makefile in case of doubts."
 	@echo
 	@exit 1
+else ifneq ($(filter $(TARGET), linux linux22 linux24 linux24e linux26 linux2628),)
+all:
+	@echo
+	@echo "Target '$(TARGET)' was removed from HAProxy 2.0 due to being irrelevant and"
+	@echo "often wrong. Please use 'linux-glibc' instead or define your custom target"
+	@echo "by checking available options using 'make help TARGET=<your-target>'."
+	@echo
+	@exit 1
 else
 all: haproxy $(EXTRA)
 endif