Add --output-sync=target to gnumake

.. if it is supported and we aren't building on mingw
where it is a bit broken.
Export build tools when restoring

.. if CT_EXPORT_BUILD_TOOLS was specified then we do
'export AR AS DLLTOOL CC CXX LD NM OBJCOPY OBJDUMP RANLIB STRIP WINDRES'
when restoring a build stage.
Add SRC_REFERENCE_DATETIME option.

If you are using a repeatable builds system such as bitcoin gitian
then you can use this option to force a specific date and time on
the source files (post patching).