[gcc r15-8027] libstdc++: Allow 'configure.host' to pre-set 'EXTRA_CFLAGS', 'EXTRA_CXX_FLAGS'

[Thomas Schwinge]

https://gcc.gnu.org/g:5967fe0dd1f3a49176740553cd147e99d6950895

commit r15-8027-g5967fe0dd1f3a49176740553cd147e99d6950895
Author: Thomas Schwinge <tschwinge@baylibre.com>
Date:   Wed Feb 26 10:13:51 2025 +0100

    libstdc++: Allow 'configure.host' to pre-set 'EXTRA_CFLAGS', 'EXTRA_CXX_FLAGS'
    
    In particular, 'GLIBCXX_ENABLE_CXX_FLAGS' shouldn't overwrite 'EXTRA_CXX_FLAGS'
    (and instead append any additional '--enable-cxx-flags=[...]').
    
            libstdc++-v3/
            * acinclude.m4 (GLIBCXX_ENABLE_CXX_FLAGS): Append to
            'EXTRA_CXX_FLAGS' any additional flags.
            * configure: Regenerate.
            * configure.host: Document 'EXTRA_CFLAGS', 'EXTRA_CXX_FLAGS'.

Diff:
---
 libstdc++-v3/acinclude.m4   | 4 +++-
 libstdc++-v3/configure      | 4 +++-
 libstdc++-v3/configure.host | 4 ++++
 3 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/libstdc++-v3/acinclude.m4 b/libstdc++-v3/acinclude.m4
index b3423d7957ac..e668d2dba274 100644
--- a/libstdc++-v3/acinclude.m4
+++ b/libstdc++-v3/acinclude.m4
@@ -3267,9 +3267,11 @@ AC_DEFUN([GLIBCXX_ENABLE_CXX_FLAGS], [dnl
 	     AC_MSG_ERROR([compiler flags start with a -]) ;;
       esac
     done
+
+    # Append the additional flags to any that came from 'configure.host'.
+    EXTRA_CXX_FLAGS="$EXTRA_CXX_FLAGS $enable_cxx_flags"
   fi
 
-  EXTRA_CXX_FLAGS="$enable_cxx_flags"
   AC_MSG_RESULT($EXTRA_CXX_FLAGS)
   AC_SUBST(EXTRA_CXX_FLAGS)
 ])
diff --git a/libstdc++-v3/configure b/libstdc++-v3/configure
index e115ee55739f..78758285f211 100755
--- a/libstdc++-v3/configure
+++ b/libstdc++-v3/configure
@@ -19450,9 +19450,11 @@ fi
 	     as_fn_error $? "compiler flags start with a -" "$LINENO" 5 ;;
       esac
     done
+
+    # Append the additional flags to any that came from 'configure.host'.
+    EXTRA_CXX_FLAGS="$EXTRA_CXX_FLAGS $enable_cxx_flags"
   fi
 
-  EXTRA_CXX_FLAGS="$enable_cxx_flags"
   { $as_echo "$as_me:${as_lineno-$LINENO}: result: $EXTRA_CXX_FLAGS" >&5
 $as_echo "$EXTRA_CXX_FLAGS" >&6; }
 
diff --git a/libstdc++-v3/configure.host b/libstdc++-v3/configure.host
index 7bc430716168..933a43f241c3 100644
--- a/libstdc++-v3/configure.host
+++ b/libstdc++-v3/configure.host
@@ -61,6 +61,10 @@
 #
 # It possibly modifies the following variables:
 #
+#   EXTRA_CFLAGS           extra flags to pass when compiling C code
+#
+#   EXTRA_CXX_FLAGS        extra flags to pass when compiling C++ code
+#
 #   OPT_LDFLAGS            extra flags to pass when linking the library, of
 #                          the form '-Wl,blah'
 #                          (defaults to empty in acinclude.m4)