[Win] DLLs are missing version information.

[Win] DLLs are missing version information.
https://bugs.webkit.org/show_bug.cgi?id=159349

Reviewed by Alex Christensen.

Avoid using environment variable WEBKIT_LIBRARIES when finding version stamper utility,
in case it is not defined. Instead, use the location of the perl script to find the
utility.

.:

* Source/cmake/tools/scripts/version-stamp.pl:

WebKitLibraries:

* win/tools/scripts/version-stamp.pl:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@203167 268f45cc-cd09-0410-ab3c-d52691b4dbfc