configurator: DEFAULT_{COMPILER, FLAGS} for MSVC

configurator: DEFAULT_{COMPILER, FLAGS} for MSVC

When compiling with Visual Studio, use default compiler name and flags
which are likely to work with the known-available compiler.

This is also a convenience for users who may not know what arguments
cl.exe may need to compile the tests.

Changes since v1:
- Use "-option" instead of "/option" to avoid issues running under msys.
- Disable C4200 warning for use of flexible array members, which MSVC
  considers an extension (since it does not fully support C99).

Signed-off-by: Kevin Locke <kevin@kevinlocke.name>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>