Simplify standalone Minijail compilation.

Allow LIBDIR to be set from the environment, so that Minijail can be
compiled and used in-tree.

Also add a .gitignore file.

Bug: 30276233
diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..4f62b55
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,12 @@
+*.d
+*.depends
+*.gen.c
+*.o
+
+# Shared libraries when compiling in-tree.
+*.so
+
+# Executables when compiling in-tree.
+libminijail_unittest
+minijail0
+syscall_filter_unittest