run-make tests should have a way to ignore targets

[euclio]

Currently, run-make targets that only run on particular targets do so by using the preprocessor:

rust/src/test/run-make-fulldeps/windows-spawn/Makefile

Lines 3 to 14 in b7c6e8f

	ifdef IS_WINDOWS
	all:
	$(RUSTC) -o "$(TMPDIR)/hopefullydoesntexist bar.exe" hello.rs
	$(RUSTC) spawn.rs
	$(TMPDIR)/spawn.exe
	else
	all:
	endif

This hurts readability and causes the tests to show up as "passed" on platforms where they aren't run. It would be nice if these tests could write a comment such as # ignore-windows or # only-windows and have the tests show up as ignored, like UI tests.