CMakeLists.txt should not force -stdlib=libc++

[Olipro]

When building cpprestsdk with Clang, it is entirely plausible that you may wish to compile against libstdc++ instead of libc++.

Consequently, I suggest that the CMakeLists.txt either omit the addition of the option, make it a configurable cache option or, first detect if the builder has already inserted a desired stdlib into the CMAKE_CXX_FLAGS variable and avoid appending -lstdib=libc++ in that case.

[fcharlie]

This PR #465 Fix build on linux auto add libc++