Add support for SANE_CONFIG variable

Instead of using directly the sane-config command, allow to pass a
SANE_CONFIG environment variable to override where to find the
sane-config tool.

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

Index: b/configure.ac
===================================================================
--- a/configure.ac
+++ b/configure.ac
@@ -1375,7 +1375,7 @@
 if test "x$with_sane" != "xno"
 then
     ac_save_CPPFLAGS="$CPPFLAGS"
-    WINE_PACKAGE_FLAGS(SANE,[libsane],,[`sane-config --cflags 2>/dev/null`],[`sane-config --ldflags 2>/dev/null`])
+    WINE_PACKAGE_FLAGS(SANE,[libsane],,[`${SANE_CONFIG:-sane-config} --cflags 2>/dev/null`],[`${SANE_CONFIG:-sane-config} --ldflags 2>/dev/null`])
     AC_CHECK_HEADER(sane/sane.h,
         [WINE_CHECK_SONAME(sane,sane_init,,[SANE_CFLAGS=""],[$SANE_LIBS])],
         [SANE_CFLAGS=""])