Add additional gitignore pattern for build directories.

Related to #9
diff --git a/.gitignore b/.gitignore
index 95ef78e..025558d 100644
--- a/.gitignore
+++ b/.gitignore
@@ -15,6 +15,7 @@
 
 # CMake artifacts
 build/
+build-*/
 
 # Bazel artifacts
 /bazel-*