genVSproperties.bat: why unset variables?

[Ilya Kazakevich]

Hello.

I was trying to build jfx for Windows using cygwin.
I launched cygwin from VS prompt with all variables set, but when build
failed,
I realized that ``windows_tools.properties`` is almost empty.

It happened because ``genVSproperties.bat`` failed to locate my toolchain
and
it explicitly removed my variables!

REM Clean out the current settings
set INCLUDE=
set LIB=
set LIBPATH=

Why so? Is not it better to remove this lines and always launch build via
VS prompt?
After all, this is what it was created for)

Ilya.