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>

diff --git a/Makefile b/Makefile
index 3d90488dab7f28b37a96a81ff6f402455ed7322e..596f0268ba94145a73c42d22aaa0b8ea510c3b77 100644 (file)
--- a/Makefile
+++ b/Makefile
@@ -1366,12 +1366,12 @@ else
        endif
 
        ifndef CURL_LDFLAGS
-               CURL_LDFLAGS := $(shell $(CURL_CONFIG) --libs)
+               CURL_LDFLAGS = $(eval CURL_LDFLAGS := $$(shell $$(CURL_CONFIG) --libs))$(CURL_LDFLAGS)
        endif
        CURL_LIBCURL += $(CURL_LDFLAGS)
 
        ifndef CURL_CFLAGS
-               CURL_CFLAGS := $(shell $(CURL_CONFIG) --cflags)
+               CURL_CFLAGS = $(eval CURL_CFLAGS := $$(shell $$(CURL_CONFIG) --cflags))$(CURL_CFLAGS)
        endif
        BASIC_CFLAGS += $(CURL_CFLAGS)