Make OTP installation optional (#81)

test-projects/gleam_gleam/gleam.toml

@@ -0,0 +1,16 @@
+name = "gleam_gleam"
+version = "0.1.0"
+
+# Fill out these fields if you intend to generate HTML documentation or publish
+# your project to the Hex package manager.
+#
+# licences = ["Apache-2.0"]
+# description = "A Gleam library..."
+# repository = { type = "github", user = "username", repo = "project" }
+# links = [{ title = "Website", href = "https://gleam.run" }]
+
+[dependencies]
+gleam_stdlib = "~> 0.18"
+
+[dev-dependencies]
+gleeunit = "~> 0.5"