Makefile: avoid running curl-config multiple times

If the user hasn't set the CURL_LDFLAGS Makefile variable, we invoke curl-config like this: CURL_LIBCURL += $(shell $(CURL_CONFIG) --libs) Because the shell function is run when the value is expanded, we invoke curl-config each time we need to link something (which generally ends up being four times for a full build). Instead, let's use an immediate Makefile variable, which only needs expanding once. We can't combine that with the existing "+=", but since we only do this when CURL_LDFLAGS is undefined, we can just set that variable. That also allows us to simplify our conditional a bit, since both sides will then put the result into CURL_LIBCURL. While we're touching it, let's fix the indentation to match the nearby code (we're inside an outer conditional, so everything else is indented one level). Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>