fix build on linux use clang

[fcharlie]

When use clang build on cpprestsdk on Linux, The original CMakeLists will automatically add the parameter -stdlib=c++ if boost is not built using libc++, then the entire project will fail to build.

For almost all Linux distributions,When the boost that is installed through the Package Manager is using GNU libstdc++ and not clang libc++.

This PR will remove -stdlib=libc++ on linux.

Thanks all.

[msftclas]

@fcharlie,
Thanks for your contribution.
To ensure that the project team has proper rights to use your work, please complete the Contribution License Agreement at https://cla.microsoft.com.

It will cover your contributions to all Microsoft-managed open source projects.
Thanks,
Microsoft Pull Request Bot

[fcharlie]

signed the CLA

[msftclas]

@fcharlie, thanks for signing the contribution license agreement. We will now validate the agreement and then the pull request.

Thanks, Microsoft Pull Request Bot

[ras0219-msft]

Thanks, looks good.