[osg] fix non-windows builds

[Outurnate]

Enabling OSG_USE_UTF8_FILENAME on any platform other than windows causes the build to fail. This is because it substitutes fopen for _wfopen, which is only on windows.

Related: #9449.

[NancyLi1013]

Hi @Outurnate thanks for this PR.
What issue does this PR fix? Please open an issue so others may replicate this problem.

[Outurnate]

Fixes #9446

All requested changes made. Some automated checks had recommendations as well. There were also some windows-centric bits that I fixed

[NancyLi1013]

It seems that #9446 has nothing with this PR.
Are you sure that this PR fixes #9446?

[Outurnate]

Sorry, no, it's #9449

[NancyLi1013]

Since osgearth depends on osg. Please also fix this issue on osgearth.

Building package osgearth[core]:x86-windows...
Could not locate cached archive: C:\vsts\_work\3\s\archives\4b\4b94742375b9025d6141efb2462e4db7f333f788.zip
CMake Error at ports/osgearth/portfile.cmake:8 (message):
  Could not determine osg version
Call Stack (most recent call first):
  scripts/ports.cmake:94 (include)

[NancyLi1013]

/azp run

[Outurnate]

I'll need to modify osg to build the tools again. osgearth depends on it

[NancyLi1013]

/azp run

[NancyLi1013]

Could you please resolve the conflicts?

[NancyLi1013]

@Outurnate
Is work still being done for this PR?

[NancyLi1013]

Thanks for the PR; we're closing this for now since there's been no response. If you'd like to continue working on it, please reopen and ping us!