build: fix compatibility with rustc 1.30.0

Additionally:
* Rebuild rust crates when the rustc version changes.
* Fetch all rust ldflags in one exec_script() call instead of two.

1 files changed, 1 insertions, 0 deletions

diff --git a/build_extra/rust/empty_crate.rs b/build_extra/rust/empty_crate.rs
new file mode 100644
index 000000000..f328e4d9d
--- /dev/null
+++ b/build_extra/rust/empty_crate.rs
@@ -0,0 +1 @@
+fn main() {}