CMake/Windows: Compile GMP library as fat library

Windows: Fix build issue on windows

TBB includes Windows.h which defines a min/max macro
leading to issues when you want to use std::min and
std::max.

This change prevents Windows.h from defining them
sidestepping the issue.

Cleanup: Fix build error on windows

int and eContextResult are different types to MSVC
leading to a function signature mismatch.

Cycles: Fix broken 32 bit shift.

1ul << n will still be a 32 bit integer regardless
of the value of n, given the target here is 64 bits
the upper 32 bits will always be zero. Using 1ull
will yield the expected result.

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