[lit] Work around issue where can't set the host library environment.

Author: DougGregor

test/lit.cfg

@@ -2480,9 +2480,13 @@ config.substitutions.append(('%llvm-profdata', config.llvm_profdata))
 config.substitutions.append(('%llvm-cov', config.llvm_cov))
 
 # Set up the host library environment.
-host_library_env = (
-        "/usr/bin/env " +
-        construct_library_path_env(config.swift_host_lib_dir))
+if hasattr(config, 'target_library_path_var'):
+  host_library_env = (
+          "/usr/bin/env " +
+          construct_library_path_env(config.swift_host_lib_dir))
+else:
+  host_library_env = ""
+
 config.substitutions.append(('%host-library-env', host_library_env))
 
 if hasattr(config, 'otool_classic'):