In Makerules somewhere around line 1097 there is the following fragment.

$(addprefix $(inst_includedir)/,$(headers)): $(inst_includedir)/%: % $(+force)
	echo "doing header installation"
	$(do-install)

When configured with --enable-force-install +force variable contains .PHONY
target force-install. This makes $(addprefix $(inst_includedir)/,$(headers))
always outdated which makes make always try to rebuild them and also (here is
the catch) makes anything that depends on any header from $(addprefix
$(inst_includedir)/,$(headers)) outdated too.

Now let's take a look at tls.make.c or rather at the makefile fragment
(tls.make) that get's generated from it. There we see something like this:

tls.make: some-path/limits.h

And now the last piece of the puzzle from Makerules:

-include $(common-objpfx)tls.make

Putting it all together gives the following effect:

1. make tries to include tls.make - it does not exist

2. but there is a rule to build it from tls.make.c so make does that

3. make re-executes itself and reads tls.make in

4. inside tls.make there is a rule that says tls.make depends on limits.h

5. and there is a rule how to build limits.h which also makes limits.h always
   outdated

6. make has no choice but to rebuild limits.h and as a result tls.make
  
7. now make has to re-execute again because tls.make has changed, so goto (3)

The bottom line: depending real tragets (files) on phony targets is a bad idea ;-)

hth,
-boris