Hide cmake cache during fakebuild, so that the previous in-tree build…

…s do not affect build system generation.

See ycm-core#16

config_gen.py

@@ -182,6 +182,17 @@ def run(cmd, *args, **kwargs):
         build_dir = tempfile.mkdtemp()
         proc_opts["cwd"] = build_dir
 
+        # if the project was built in-tree, we need to hide the cache file so that cmake
+        # populates the build dir instead of just re-generating the existing files
+        cache_path = os.path.join(project_dir, "CMakeCache.txt")
+
+        if(os.path.exists(cache_path)):
+            fd, cache_tmp = tempfile.mkstemp()
+            os.close(fd)
+            shutil.move(cache_path, cache_tmp)
+        else:
+            cache_tmp = None
+
         print("Running cmake in '{}'...".format(build_dir))
         run(["cmake", project_dir] + configure_opts, env=env_config, **proc_opts)
 
@@ -192,6 +203,9 @@ def run(cmd, *args, **kwargs):
         print("")
         shutil.rmtree(build_dir)
 
+        if(cache_tmp):
+            shutil.move(cache_tmp, cache_path)
+
     elif(os.path.exists(os.path.join(project_dir, "configure"))):
         # Autotools
         # perform build in-tree, since not all projects handle out-of-tree builds correctly