BUILD: makefile: make the obsolete target detection compatible with make-3.80

Older versions of GNU make do not support "else ifneq", let's split
this in two lines.
diff --git a/Makefile b/Makefile
index a9b21b9..785e313 100644
--- a/Makefile
+++ b/Makefile
@@ -742,7 +742,8 @@
 	@echo "out of it. Please check the Makefile in case of doubts."
 	@echo
 	@exit 1
-else ifneq ($(filter $(TARGET), linux linux22 linux24 linux24e linux26 linux2628),)
+else
+ifneq ($(filter $(TARGET), linux linux22 linux24 linux24e linux26 linux2628),)
 all:
 	@echo
 	@echo "Target '$(TARGET)' was removed from HAProxy 2.0 due to being irrelevant and"
@@ -753,6 +754,7 @@
 else
 all: haproxy $(EXTRA)
 endif
+endif
 
 OBJS = src/proto_http.o src/cfgparse-listen.o src/proto_htx.o src/stream.o    \
        src/mux_h2.o src/stats.o src/flt_spoe.o src/server.o src/checks.o      \