deb/cross: stop exporting PKG_CONFIG_LIBDIR for cross builds

dpkg-buildpackage and sbuild export no pkg-config redirection: the
environment is the package's to set.  Pointing the whole build at the
target multiarch pkgconfig dirs makes every pkg-config-consuming tool
resolve against the target libraries, the host-side tools of the same
build included: the host linker is then handed target-arch -L paths
(the kernel's resolve_btfids logging 'skipping incompatible
/usr/lib/aarch64-linux-gnu/libelf.so') and only survives through
fallbacks when the build-architecture libraries happen to be installed
too.

Packages whose cross builds need target pkg-config data must arrange
for it themselves, the way the kernel packaging builds rtla statically
against no target system libraries.

src/deb/cross.rs

@@ -62,20 +62,6 @@ pub fn setup_environment(
         .map_err(|e| format!("Invalid UTF-8 in dpkg-architecture output: {e}"))?;
     parse_dpkg_architecture_output(&dpkg_architecture, env);
 
-    // In-tree tools locate their libraries with the *host* pkg-config during
-    // cross builds (the kernel's tools/build feature checks derive their
-    // cflags/ldflags from `pkg-config --cflags/--libs`), whose search path
-    // only covers the build architecture's pkgconfig dirs. Point it at the
-    // target's so `libtraceevent` & co resolve to target-arch libraries:
-    // linux-riscv cross builds die in rtla's Makefile.config otherwise, even
-    // with the target -dev packages installed.
-    if let Some(multiarch) = env.get("DEB_HOST_MULTIARCH").cloned() {
-        env.insert(
-            "PKG_CONFIG_LIBDIR".to_string(),
-            format!("/usr/lib/{multiarch}/pkgconfig:/usr/share/pkgconfig"),
-        );
-    }
-
     env.insert("DEB_BUILD_PROFILES".to_string(), "cross".to_string());
 
     Ok(())
@@ -304,21 +290,18 @@ mod tests {
         assert!(cross_suites("noble", None, "not-a-distro").is_err());
     }
 
-    /// setup_environment exports the target multiarch pkg-config libdir:
-    /// tools' feature checks run the *host* pkg-config, which must find the
-    /// target's .pc files (rtla hard-errors on libtraceevent otherwise,
-    /// failing linux-riscv cross builds despite the target -dev packages
-    /// being installed).
+    /// setup_environment exports the dpkg cross variables and the 'cross'
+    /// build profile, and nothing beyond what dpkg-buildpackage exports:
+    /// no pkg-config redirection (the environment is the package's to
+    /// set, and target-arch pkgconfig paths would also poison the
+    /// host-side tools of the same build).
     #[test]
-    fn test_setup_environment_exports_cross_pkg_config_libdir() {
+    fn test_setup_environment_exports_dpkg_cross_variables_only() {
         let mut env = HashMap::new();
         let ctx = Arc::new(Context::new(crate::context::ContextConfig::Local).unwrap());
         setup_environment(&mut env, "riscv64", ctx).unwrap();
 
-        assert_eq!(
-            env.get("PKG_CONFIG_LIBDIR").map(String::as_str),
-            Some("/usr/lib/riscv64-linux-gnu/pkgconfig:/usr/share/pkgconfig")
-        );
+        assert!(!env.contains_key("PKG_CONFIG_LIBDIR"));
         assert_eq!(
             env.get("DEB_BUILD_PROFILES").map(String::as_str),
             Some("cross")