CMake: Enable WITH_POTRACE by default

All platforms have landed the libs, this can be on
by default now.

deps: fix build issue on windows.

Windows does not build libxml2 so the dependency recently
added for mac caused a cmake error.

GP: Update GP to use potrace from SVN/System

and remove it from /extern

Deps: Add potrace as a new library dependency

For work the GP team plans to land soon (T79877) potrace was taken
on as an additional optional dependency.

This diff adds building the library to the deps builder and takes
care of the integration into the build-system with the `WITH_POTRACE`
cmake switch.

Differential Revision: https://developer.blender.org/D8662

Reviewed by: brecht, sergey

Cleanup: Fix whitespace changes

Introduced in rB40dcf686f04f7db8110f9c85621eb8a0bd764080

Cleanup: Fix build warning with MSVC

MSVC warns on MEM_freeN being fed a const pointer

Cleanup: Fix bad spelling in previous commit.

Thanks for @deadpin for noticing!

CLog: Support colorized logging on windows

When using Windows Terminal the same control codes Linux
uses to colorize the text can be used. WT can be detected
by looking at the WT_SESSION environment variable.

Differential Revision: https://developer.blender.org/D8848

Reviewed by: campbellbarton

Cleanup: CLog fix bad use_color default

`use_color` was defaulted to 'on' and only turned off when it was
decided the terminal didn't support this. This was the wrong
way to go about things, since that test only ran for Linux/Mac other
operating systems would get color formatting regardless if they
supported it.

The new default is `off` until the platform specific code inside
`CLG_ctx_output_set` turns it on after running some sanity checks.

This fix was part of the review of D8848 but deserved its on commit

Reviewed By: campbellbarton

Cleanup: Update doc string for BLI_getenv

The reason for this functions existence was poorly documented