link: add libstdc++

Cargo.toml

@@ -18,6 +18,7 @@ name = "make_bitmap"
 path = "src/devel/make_bitmap.rs"
 
 [dependencies]
+link-cplusplus = { version = "1.0", features = ["libstdc++"] }
 geos = { version = "8" }
 lazy_static = "1.4"
 numpy = { version = "0.19" }

src/lib.rs

@@ -62,6 +62,10 @@ extern crate test;
 #[macro_use]
 extern crate lazy_static;
 
+// geos-sys needs libc++, probably libstdc++. On Windows Conda builds this hopefully adds the
+// correct flags to the linker.
+extern crate link_cplusplus;
+
 use numpy::{PyArray, PyReadonlyArrayDyn};
 use pyo3::prelude::*;
 use std::io;