libstdc++: Add dg-require-cpp-feature-test to test feature test macros

This adds a new dejagnu directive which can be used to make a test
depend on a feature test macro such as __cpp_lib_text_encoding. This is
mroe flexible than writing a new dg-require-xxx for each feature.

libstdc++-v3/ChangeLog:

	* testsuite/lib/dg-options.exp (dg-require-cpp-feature-test):
	New proc.
	* testsuite/lib/libstdc++.exp (check_v3_target_cpp_feature_test):
	New proc.
	* testsuite/std/text_encoding/cons.cc: Use new directive to skip
	the test if the __cpp_lib_text_encoding feature test macro is
	not defined.
	* testsuite/std/text_encoding/requirements.cc: Likewise.

2 files changed, 24 insertions, 0 deletions

diff --git a/libstdc++-v3/testsuite/lib/dg-options.exp b/libstdc++-v3/testsuite/lib/dg-options.exp
index 00ca678..802bfc0 100644
--- a/libstdc++-v3/testsuite/lib/dg-options.exp
+++ b/libstdc++-v3/testsuite/lib/dg-options.exp
@@ -277,6 +277,15 @@ proc dg-require-target-fs-lwt { args } {
     return
 }
 
+proc dg-require-cpp-feature-test { n args } {
+    if { ![ check_v3_target_cpp_feature_test $args ] } {
+	upvar dg-do-what dg-do-what
+	set dg-do-what [list [lindex ${dg-do-what} 0] "N" "P"]
+	return
+    }
+    return
+}
+
 proc add_options_for_no_pch { flags } {
     # Remove any inclusion of bits/stdc++.h from the options.
     regsub -all -- "-include bits/stdc...h" $flags "" flags
diff --git a/libstdc++-v3/testsuite/lib/libstdc++.exp b/libstdc++-v3/testsuite/lib/libstdc++.exp
index 7466fb5..2b31c95 100644
--- a/libstdc++-v3/testsuite/lib/libstdc++.exp
+++ b/libstdc++-v3/testsuite/lib/libstdc++.exp
@@ -1134,6 +1134,21 @@ proc v3_check_preprocessor_condition { name cond { inc "" } } {
     return [v3_try_preprocess name $code $flags]
 }
 
+# Return 1 if libstdc++ defines macro for the current target, 0 otherwise.
+proc check_v3_target_cpp_feature_test { cond } {
+    global cxxflags
+    set cxxflags_saved $cxxflags
+    # Use the latest standard, so that all feature test macros are defined.
+    # We need to do it here, because this check happens before v3-dg-runtest
+    # runs its loop to test multiple times with different -std options.
+    # This should be updated when a new -std is added.
+    set cxxflags "$cxxflags -std=gnu++26"
+    set inc "#include <version>"
+    set result [v3_check_preprocessor_condition cpp_feature_test "$cond" $inc]
+    set cxxflags $cxxflags_saved
+    return $result
+}
+
 # Return 1 if Debug Mode is active, 0 otherwise.
 proc check_v3_target_debug_mode { } {
     global cxxflags