Fix "appWithoutSources" blackbox test for Windows.

The "main" symbol seems to lose its magic if it comes from a library, so
tell the linker where the entry point is. For this purpose, use C
instead of C++, so we won't have to deal with name mangling.

Change-Id: Ia2c0714431a1b54ebcbba3d13851d4cfcf9a6c3e
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Fix access to null object in MSVC linker rules.

We no longer require object files to be present when linking.

Change-Id: I7110df35e65578964b7f4b5d12b8b10a5854cc01
Reviewed-by: Jake Petroules <jake.petroules@petroules.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>