builtin: allow Windows users to link against gc boehm

[Ekopalypse]

If this does not make sense from the project's point of view, please just close it.

This workaround allows Windows users to link against gc boehm with self-compiled shared or static gc boehm libs.
To do this, one just needs to add an additional flag to the v-source, such as.

$if msvc {
	// #flag -lD:\\ProgramData\\Compiler\\gc-8.0.4\\gc64_dll  // 64bit dll usage
	#flag -lD:\\ProgramData\\Compiler\\gc-8.0.4\\gc        // static lib usage
}

fn main() { 
	println('I was build with boehm')
}