Allow running configure only

[jonhoo]

Currently it's possible to skip the "target" step (install by default), but it's not possible to skip the cmake --build invocation (i.e., to only run configure). It'd be nice if there was something like .configure() as a dual to .build() that exited right after configure is complete here:

cmake-rs/src/lib.rs

Lines 812 to 817 in c4a60dd

	if self.always_configure || !build.join("CMakeCache.txt").exists() {
	cmd.args(&self.configure_args);
	run(cmd.env("CMAKE_PREFIX_PATH", cmake_prefix_path), "cmake");
	} else {
	println!("CMake project was already configured. Skipping configuration step.");
	}

The motivation here is for crates that only need headers to be put in the right place, but don't necessarily need the underlying library to be built, such as if they just need to invoke bindgen: jonhoo/rust-ibverbs#36