[cmake] root wrapper

allow the existence of a `CMakeLists.txt` file at root,
for easier integration with other projects expecting this file at root.

Existing integration point, within `build/cmake/`, still works as expected.

.gitignore

@@ -22,16 +22,6 @@ zstdmt
 *.out
 *.app
 
-# Test artefacts
-tmp*
-*.zst
-*.zstd
-dictionary.
-dictionary
-NUL
-cmakebuild/
-install/
-
 # Build artefacts
 contrib/linux-kernel/linux/
 projects/
@@ -40,6 +30,17 @@ bin/
 buck-out/
 build-*
 *.gcda
+cmakebuild/
+cmake-build/
+
+# Test artefacts
+tmp*
+*.zst
+*.zstd
+dictionary.
+dictionary
+NUL
+install/
 
 # IDE
 .clang_complete