Add a test case to ensure that `CPPFLAGS`, `CFLAGS`, and `LDSHARED`
flags captured during `./configure` are present when customizing the
compiler (for Unix).

Since preprocessor flags may be specified via `CPPFLAGS` instead of
`CFLAGS` during `./configure`, ensure that the `CPPFLAGS` are added to
the appropriate executables when customizing the compiler via
`sysconfig`.