fix compilation with nixpkgs 21.05

The environment variable is optional to keep compatibility with other build environments.

Closes #131

default.nix

@@ -31,6 +31,7 @@ let
       ];
       buildPhase = ''
         export XARGO_RUST_SRC="${rustPlatform.rust.rustc}/lib/rustlib/src/rust/library"
+        export CLANG_EXTRA_INCLUDE_DIR="${pkgs.llvmPackages_9.clang-unwrapped.lib}/lib/clang/9.0.1/include"
         export CARGO_HOME=$(mktemp -d cargo-home.XXX)
         make TARGET=${target} GWARGS="${if json == null then "-V ${variant}" else json}"
       '';

shell.nix

@@ -30,6 +30,7 @@ in
     ];
 
     XARGO_RUST_SRC = "${rustPlatform.rust.rustc}/lib/rustlib/src/rust/library";
+    CLANG_EXTRA_INCLUDE_DIR = "${pkgs.llvmPackages_9.clang-unwrapped.lib}/lib/clang/9.0.1/include";
     OPENOCD_ZYNQ = "${zynq-rs}/openocd";
     SZL = "${zc706-szl}/szl.elf";
   }

src/libc/build.rs

@@ -5,6 +5,8 @@ fn main() {
 
 mod libc {
     use std::path::Path;
+    use std::env;
+
     pub fn compile() {
         let cfg = &mut cc::Build::new();
         cfg.no_default_flags(true);
@@ -16,6 +18,9 @@ mod libc {
         cfg.flag("-ffreestanding");
         cfg.flag("-fno-PIC");
         cfg.flag("-isystem../include");
+        if let Ok(extra_include) = env::var("CLANG_EXTRA_INCLUDE_DIR") {
+            cfg.flag(&("-isystem".to_owned() + &extra_include));
+        }
         cfg.flag("-fno-stack-protector");
         cfg.flag("--target=armv7-none-eabihf");
         cfg.flag("-O2");

src/libunwind/build.rs

@@ -6,6 +6,7 @@ fn main() {
 
 mod llvm_libunwind {
     use std::path::Path;
+    use std::env;
 
     fn setup_options(cfg: &mut cc::Build) {
         cfg.no_default_flags(true);
@@ -16,6 +17,9 @@ mod llvm_libunwind {
         cfg.flag("-fno-PIC");
         cfg.flag("-Isrc");
         cfg.flag("-isystem../include");
+        if let Ok(extra_include) = env::var("CLANG_EXTRA_INCLUDE_DIR") {
+            cfg.flag(&("-isystem".to_owned() + &extra_include));
+        }
         cfg.flag("-fno-stack-protector");
         cfg.flag("--target=armv7-none-eabihf");
         cfg.flag("-O2");