doc: fix Interconnect example documentation.
package: added MinGW-w64 Jenkins CI configuration.

Also added missing NaCl configuration to documentation.
Added CORRADE_IS_DEBUG_BUILD preprocessor variable.

Defined by CMake on multi-configuration build systems if given build
configuration is Debug. Not defined in any case on single-configuration
build sytems.
Be consistent and use our own platform definitions.
Added CORRADE_TARGET_APPLE CMake and preprocessor variable.

To replace CMake's APPLE and preprocessor __APPLE__ with some
consistency.