ndk-build: Switch NDK r23 -lgcc workaround to `CARGO_ENCODED_RUSTFLAG…

…S` env var for paths with spaces

On Windows, user paths usually include people's full name, with a space.
If they check out a git repository under this name `cargo` fails to
parse the workaround path:

    process didn't exit successfully: `rustc - --crate-name ___ --print=file-names -L '"C:\Users\Bla' 'Haj\Desktop\Breda\target\cargo-apk-temp-extra-link-libraries"' ... (exit code: 1)

As per the [documentation for `RUSTFLAGS`] it is split by a very simple
`.split(" ")` argument without caring for quotation delimiters or escape
characters. To retain backwards compatibility this has been implemented
in a new environment variable named `CARGO_ENCODED_RUSTFLAGS`, which
separates by ASCII Unit Separators (`0x1f`)instead.

[documentation for `RUSTFLAGS`]: https://doc.rust-lang.org/cargo/reference/environment-variables.html

ndk-build/src/cargo.rs

@@ -41,18 +41,21 @@ pub fn cargo_ndk(
         // forwarded to the final compiler invocation rendering our workaround ineffective.
         // The cargo page documenting this discrepancy (https://doc.rust-lang.org/cargo/commands/cargo-rustc.html)
         // suggests to resort to RUSTFLAGS, which are updated below:
-        let mut rustflags = match std::env::var("RUSTFLAGS") {
+        let mut rustflags = match std::env::var("CARGO_ENCODED_RUSTFLAGS") {
             Ok(val) => val,
             Err(std::env::VarError::NotPresent) => "".to_string(),
             Err(std::env::VarError::NotUnicode(_)) => {
                 panic!("RUSTFLAGS environment variable contains non-unicode characters")
             }
         };
-        rustflags += " -L ";
+        if !rustflags.is_empty() {
+            rustflags.push('\x1f');
+        }
+        rustflags += "-L\x1f";
         rustflags += cargo_apk_link_dir
             .to_str()
             .expect("Target dir must be valid UTF-8");
-        cargo.env("RUSTFLAGS", rustflags);
+        cargo.env("CARGO_ENCODED_RUSTFLAGS", rustflags);
     }
 
     Ok(cargo)