check for span in libstdc++ rather than libc++ on linux

Author: hnrklssn

test/Interop/lit.local.cfg

@@ -44,18 +44,23 @@ else:
     config.substitutions.insert(0, ('%target-abi', 'SYSV'))
     config.substitutions.insert(0, ('%target-swift-flags', ''))
 
-# target_sdk_libcxx_path is empty when targeting non-Darwin platforms
-config.target_libcxx_path = config.target_sdk_libcxx_path
-
 if get_target_os() in ['linux-gnu']:
-    for p in ['/usr/include/c++/v1', '/usr/local/include/c++/v1']:
-        if os.path.exists(p) or os.path.exists(p):
-            config.available_features.add('system_wide_libcxx')
-            config.target_libcxx_path = p
+    if os.path.exists('/usr/include/c++/v1') or os.path.exists('/usr/local/include/c++/v1'):
+        config.available_features.add('system_wide_libcxx')
 
-if config.target_libcxx_path != '':
+if config.target_sdk_libcxx_path != '':
     if os.path.exists(os.path.join(config.target_sdk_libcxx_path, "span")):
         config.available_features.add('std_span')
+elif get_target_os() in ['linux-gnu']:
+    for p in ['/usr/include/c++/', '/usr/local/include/c++/']:
+        if not os.path.isdir(p):
+            continue
+        for subdir in os.listdir(p):
+            if not subdir.isdigit():
+                # skip paths libc++ paths like /usr/include/c++/v1, we want libstdc++ paths only (like /usr/include/c++/13)
+                continue
+            if os.path.exists(os.path.join(p, subdir, "span")):
+                config.available_features.add('std_span')
 elif get_target_os() in ['windows-msvc']:
     # We don't test on any versions of MSVC without std::span support.
     # FIXME: figure out where to do lookup for C++ stdlib headers on Windows - we'll probably need it eventually