Makefile: avoid running curl-config unnecessarily

Makefile: avoid running curl-config unnecessarily

Commit 94a88e2524 (Makefile: avoid running curl-config multiple times,
2020-03-26) put the call to $(CURL_CONFIG) into a "simple" variable
which is expanded immediately, rather than expanding it each time it's
needed. However, that also means that we expand it whenever the Makefile
is parsed, whether we need it or not.

This is wasteful, but also breaks the ci/test-documentation.sh job, as
it does not have curl at all and complains about the extra messages to
stderr. An easy way to see it is just:

  $ make CURL_CONFIG=does-not-work check-builtins
  make: does-not-work: Command not found
  make: does-not-work: Command not found
  GIT_VERSION = 2.26.0.108.gb3f3f45f29
  make: does-not-work: Command not found
  make: does-not-work: Command not found
  ./check-builtins.sh

We can get the best of both worlds if we're willing to accept a little
Makefile hackery. Courtesy of the article at:

  http://make.mad-scientist.net/deferred-simple-variable-expansion/

this patch uses a lazily-evaluated recursive variable which replaces its
contents with an immediately assigned simple one on first use.

Reported-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>