Makefile: default to -lcurl when no CURL_CONFIG or CURLDIR

The original implementation of CURL_CONFIG support did not match the
original behavior of using -lcurl when CURLDIR was not set. This broke
implementations that were lacking curl-config but did have libcurl
installed along system libraries, such as MSysGit. In other words, the
assumption that curl-config is always installed was incorrect.

Instead, if CURL_CONFIG is empty or returns an empty result (e.g. due
to curl-config being missing), use the old behavior of falling back to
-lcurl.

Signed-off-by: Dave Borowitz <dborowitz@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Merge branch 'jk/external-diff-use-argv-array' (early part)

Crash fix for codepath that miscounted the necessary size for an
array when spawning an external diff program.

* 'jk/external-diff-use-argv-array' (early part):
  run_external_diff: use an argv_array for the command line
Merge branch 'db/make-with-curl'

It turns out that some platforms do ship without curl-config even
though they build with the hardcoded default -lcurl and rely on it
to work.

* db/make-with-curl:
  Makefile: default to -lcurl when no CURL_CONFIG or CURLDIR
Merge branch 'master' into next

* master:
  Makefile: default to -lcurl when no CURL_CONFIG or CURLDIR