make current BUILD-SNAPSHOT build and run under windows

for reference: http://grails.1312388.n4.nabble.com/cannot-build-grails-core-master-on-windows-td4651083.html

the build.gradle modifications allow a successful build on windows.

i had one more error with run.app: DefaultGrailsApplication tried to get the pluginManager from an unrefreshed context. this looks like coming from a reloading-background (GRAILS-10573). but at least on the initial start of the app (which is where the context is not-yet refreshed), the context gets set on the pluginManager in GrailsRuntimeConfigurator afterwards (when its refreshed).

because now i have tested these changes again only on windows, it would be really cool if a core dev could give this a try :)

thanks, zyro