os/linux/ld: handle nonexistent ld.so.conf more gracefully

Fixes #18458

Library/Homebrew/os/linux/ld.rb

@@ -23,14 +23,19 @@ def self.brewed_ld_so_diagnostics
 
       sig { returns(String) }
       def self.sysconfdir
-        fallback_sysconfdir = "/etc"
-
         match = brewed_ld_so_diagnostics.match(/path.sysconfdir="(.+)"/)
         return fallback_sysconfdir unless match
 
         match.captures.compact.first || fallback_sysconfdir
       end
 
+      sig { returns(String) }
+      def self.fallback_sysconfdir
+        return "/usr/share/defaults/etc" if File.exist?("/usr/share/defaults/etc/ld.so.conf")
+
+        "/etc"
+      end
+
       sig { returns(T::Array[String]) }
       def self.system_dirs
         dirs = []
@@ -45,9 +50,21 @@ def self.system_dirs
         dirs
       end
 
+      FALLBACK_LIBRARY_PATHS = %w[
+        /usr/local/lib
+        /usr/local/lib/x86_64-linux-gnu
+        /lib/x86_64-linux-gnu
+        /usr/lib/x86_64-linux-gnu
+      ].freeze
+
       sig { params(conf_path: T.any(Pathname, String)).returns(T::Array[String]) }
       def self.library_paths(conf_path = Pathname(sysconfdir)/"ld.so.conf")
         conf_file = Pathname(conf_path)
+        unless conf_file.exist?
+          opoo "Could not find your `ld.so.conf`; Falling back to built-in defaults."
+          return FALLBACK_LIBRARY_PATHS
+        end
+
         paths = Set.new
         directory = conf_file.realpath.dirname