Skip CMake configure step when CMakeCache.txt is present in build folder

When configuring big projects, not re-running CMake every time improves
the `cargo build` user experience.

Author: gferon

src/lib.rs

@@ -496,7 +496,10 @@ impl Config {
             cmd.env(k, v);
         }
 
-        run(cmd.env("CMAKE_PREFIX_PATH", cmake_prefix_path), "cmake");
+        if !build.join("CMakeCache.txt").exists() {
+            println!("CMake project was already configured. Skipping configuration step.");
+            run(cmd.env("CMAKE_PREFIX_PATH", cmake_prefix_path), "cmake");
+        }
 
         let mut makeflags = None;
         let mut parallel_args = Vec::new();