Makefile: make curl-config path configurable

Makefile: make curl-config path configurable

There are situations, e.g. during cross compilation, where curl-config
program is not present in the PATH.

Make the makefile use a configurable curl-config program passed through
CURL_CONFIG variable which can be set through config.mak.

Also make this variable tunable through use of autoconf/configure. Configure
will set CURL_CONFIG variable in config.mak.autogen to whatever value has been
passed to ac_cv_prog_CURL_CONFIG.

Signed-off-by: Remi Pommarel <repk@triplefau.lt>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>