Remember to AC_SUBST the Rust compiler environment variables so they're visible in Makefiles.

configure.ac

@@ -54,10 +54,12 @@ AC_CHECK_PROG(CARGO, [cargo], [yes], [no])
 AS_IF(test x$CARGO = xno,
     AC_MSG_ERROR([cargo is required.  Please set the CARGO environment variable or install the Rust toolchain from https://www.rust-lang.org/])
 )
+AC_SUBST(CARGO, [cargo])
 AC_CHECK_PROG(RUSTC, [rustc], [yes], [no])
 AS_IF(test x$RUSTC = xno,
     AC_MSG_ERROR([rustc is required.  Please set the RUSTC environment variable or install the Rust toolchain from https://www.rust-lang.org/])
 )
+AC_SUBST(RUSTC, [rustc])
 
 dnl libtool library versioning
 dnl ==========================