Use correct curl-config program

Instead of calling directly curl-config in src/Makefile (which ends up
calling the wrong curl-config: the one in the PATH instead of the one
pointed at by the environment variables at configure time), let's
define a CURL_CONFIG variable that contains the path to the proper
curl-config program, and use it where appropriate.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Index: b/src/Makefile
===================================================================
--- a/src/Makefile
+++ b/src/Makefile
@@ -57,7 +57,7 @@
   TRANSPORT_MODS += $(BLDDIR)/lib/curl_transport/xmlrpc_curl_transport
   TRANSPORT_MODS += $(BLDDIR)/lib/curl_transport/curltransaction
   TRANSPORT_MODS += $(BLDDIR)/lib/curl_transport/curlmulti
-  TRANSPORT_LIBDEP += $(shell curl-config --libs)
+  TRANSPORT_LIBDEP += $(shell $CURL_CONFIG --libs)
   TRANSPORT_INCLUDES += -Isrcdir/lib/curl_transport
 endif
 ifeq ($(MUST_BUILD_LIBWWW_CLIENT),yes)
Index: b/config.mk.in
===================================================================
--- a/config.mk.in
+++ b/config.mk.in
@@ -32,6 +32,7 @@
 LSOCKET = @LSOCKET@
 WININET_LDADD = @WININET_LDADD@
 WININET_LIBDIR = @WININET_LIBDIR@
+CURL_CONFIG = @CURL_CONFIG@
 CURL_LDADD = @CURL_LDADD@
 CURL_LIBDIR = @CURL_LIBDIR@
 LIBWWW_LDADD = @LIBWWW_LDADD@
Index: b/configure.in
===================================================================
--- a/configure.in
+++ b/configure.in
@@ -550,6 +550,8 @@
     dnl So we don't do any check now.  If we find out there's a problem with
     dnl older Curls, we will revisit that.
 
+    AC_SUBST(CURL_CONFIG)
+
     CURL_LDADD=$($CURL_CONFIG --libs)
     AC_SUBST(CURL_LDADD)
 
Index: b/lib/curl_transport/Makefile
===================================================================
--- a/lib/curl_transport/Makefile
+++ b/lib/curl_transport/Makefile
@@ -29,7 +29,7 @@
 
 $(SRCDIR)/common.mk: srcdir blddir
 
-CURL_VERSION := $(shell curl-config --vernum)
+CURL_VERSION := $(shell $(CURL_CONFIG) --vernum)
 
 # Some time at or before Curl 7.12, <curl/types.h> became an empty file
 # (no-op).  Some time after Curl 7.18, <curl/types.h> ceased to exist.
@@ -43,7 +43,7 @@
   CFLAGS_LOCAL += -DNEED_CURL_TYPES_H
 endif
 
-CURL_INCLUDES := $(shell curl-config --cflags)
+CURL_INCLUDES := $(shell $(CURL_CONFIG) --cflags)
 # We expect that curl-config --cflags just gives us -I options, because
 # we need just the -I options for 'make dep'.  Plus, it's scary to think
 # of what any other compiler flag would do to our compile.