curl tool was using functions curlx_tvnow and curlx_tvdiff which are not

curl tool was using functions curlx_tvnow and curlx_tvdiff which are not
part of the official libcurl API http://curl.haxx.se/lxr/source/lib/README.curlx
The documented way of using them would be to use timeval.c as a source code file.

The above described method works very well when statically linking libcurl and
apps, curl tool, but has several drawbacks when you build a true shared
libcurl (i.e. Name space clash at linkage stage as functions are defined more
than once. Windows makefiles are not capable of handling this system of
source-level sharing)

So...

Now curlutil.h and curlutil.c define and implement cutil_tvnow and cutil_tvdiff
which replace curlx_tvnow and curlx_tvdiff for the curl tool. Doing this we
avoid the above described problems.