Fix build issue when MAINTAINER_MODE=OFF / not set

It appears that when MAINTAINER_MODE=ON, the change in that environment
variable for the demorust ( lib/rust ) module causes the build.rs script
to be re-run.  I think that's because the MAINTAINER_MODE variable may
not be passed to `cargo test` from `ctest`, while it is passed to `cargo
build` from `cmake`.  However, when MAINTAINER_MODE=OFF there is no
change in the environment variables passed and so `build.rs` is not
re-run for `cargo test`.  As a consequence, it never links the test
executable with the required librar(ies).

The primary change in this commit to fix that is to move:
```rust
println!("cargo:rerun-if-env-changed=LIBDEMO");
```
out of the "test" match case so it applies in general.
That seems to fix it. I'm ... not 100% sure why, but it works for me!

app_rust/build.rs

@@ -81,7 +81,7 @@ fn main() -> Result<(), &'static str> {
 /// Use bindgen to generate Rust bindings to call into C libraries.
 fn generate_rust_bindings() -> Result<(), &'static str> {
     let build_dir = PathBuf::from(env::var("CARGO_TARGET_DIR").unwrap_or_else(|_| ".".into()));
-    let build_include_path = format!("-I{}", build_dir.join("..").to_str().unwrap());
+    let build_include_path = format!("-I{}", build_dir.join(".").to_str().unwrap());
 
     // Configure and generate bindings.
     let mut builder = builder()
@@ -115,6 +115,8 @@ fn generate_rust_bindings() -> Result<(), &'static str> {
         .write_to_file(BINDGEN_OUTPUT_FILE)
         .expect("Failed to write Rust bindings to output file");
 
+    eprintln!("bindgen outputting \"{}\"", BINDGEN_OUTPUT_FILE);
+
     Ok(())
 }
 

lib/rust/build.rs

@@ -41,6 +41,9 @@ fn main() -> Result<(), &'static str> {
     // FindRust.cmake defines $CARGO_CMD so we can differentiate.
     let cargo_cmd = env::var("CARGO_CMD").unwrap_or_else(|_| "".into());
 
+    // If this environmment variable chnages, we should re-run this script.
+    println!("cargo:rerun-if-env-changed=LIBDEMO");
+
     match cargo_cmd.as_str() {
         "build" => {
             // Generate bindings as a part of the build.
@@ -61,8 +64,6 @@ fn main() -> Result<(), &'static str> {
 
         "test" => {
             // Link test executable with library dependencies.
-            println!("cargo:rerun-if-env-changed=LIBDEMO");
-
             for var in LIB_ENV_LINK {
                 if !search_and_link_lib(var)? {
                     eprintln!("Undefined library dependency environment variable: {}", var);
@@ -88,7 +89,7 @@ fn main() -> Result<(), &'static str> {
 /// Use bindgen to generate Rust bindings to call into C libraries.
 fn generate_rust_bindings() -> Result<(), &'static str> {
     let build_dir = PathBuf::from(env::var("CARGO_TARGET_DIR").unwrap_or_else(|_| ".".into()));
-    let build_include_path = format!("-I{}", build_dir.join("..").to_str().unwrap());
+    let build_include_path = format!("-I{}", build_dir.join(".").to_str().unwrap());
 
     // Configure and generate bindings.
     let mut builder = builder()
@@ -122,6 +123,8 @@ fn generate_rust_bindings() -> Result<(), &'static str> {
         .write_to_file(BINDGEN_OUTPUT_FILE)
         .expect("Failed to write Rust bindings to output file");
 
+    eprintln!("bindgen outputting \"{}\"", BINDGEN_OUTPUT_FILE);
+
     Ok(())
 }