SmartLink should propagate c++ detection along library nodes

[acmorrow]

Describe the Feature
SmartLink should propagate c++ detection along library nodes

Required information

• Link to SCons Users thread discussing your issue.
  @bdbaddog asked on discord for me to file this issue.

• Version of SCons
  3.1.2

• Version of Python
  3.6

Additional context
If you have a .Program that is made up only of C source files, and it depends on a StaticLibrary node that is made of C++ files (but, say, exports a C ABI like a malloc replacement), then SmartLink will decide to use the C compiler to drive the link of the Program. This will fail though, because the necessary C++ runtime bits weren't included to satisfy the needs of the C++ compiled files.

This only affects projects which build .Programs out of StaticLibrary nodes. However, that is a fairly common setup, so probably worth fixing.

[bdbaddog]

That thread isn't something publicly viewable. Unfortunately.
It'd be sufficient to just say bdbaddog asked on discord for me to file this issue.