Regtested cris-elf, both an older newlib (FWIW: before the
getentropy issue that I hoped to investigate before
summer...maybe next summer) and a fresh checkout, both
with/without --enable-newlib-iconv.  I'm pleasantly
surprised that it works (there are no regressions) with
newlib iconv enabled compared to without: I had to
double-check the different libstdc++-v3/config.log that it
actually *was* enabled.

Ok to commit?

-- >8 --
Support for iconv in newlib seems to have been always
assumed present by libstdc++-v3, but is default off.

Though, it hasn't been used before recent libstdc++ changes
that actually call iconv functions.  This now leads to
failures exposed by running the test-suite, unless the
newlib being used has been explicitly configured with
--enable-newlib-iconv.  When failing, there are undefined
references to iconv, iconv_open or iconv_close for multiple
tests.

Thankfully there's a macro in newlib.h that we can check to
detect presence of iconv support for the newlib build that's
used.

libstdc++-v3:
        PR libstdc++/116362
        * configure.ac: Check newlib configuration whether iconv is enabled.
        * configure: Regenerate.
---
 libstdc++-v3/configure    | 26 +++++++++++++++++++++++++-
 libstdc++-v3/configure.ac | 10 +++++++++-
 2 files changed, 34 insertions(+), 2 deletions(-)

diff --git a/libstdc++-v3/configure.ac b/libstdc++-v3/configure.ac
index ccb24a82be79..4049f54bd5a3 100644
--- a/libstdc++-v3/configure.ac
+++ b/libstdc++-v3/configure.ac
@@ -376,7 +376,15 @@ dnl # rather than hardcoding that information.
       frexpl hypotl ldexpl log10l logl modfl powl sinhl sinl sqrtl
       tanhl tanl])
 
-    AC_DEFINE(HAVE_ICONV)
+    # Support for iconv in newlib is configurable.
+    AC_TRY_COMPILE([#include <newlib.h>], [
+      #ifndef _ICONV_ENABLED
+      #error
+      #endif], [ac_newlib_iconv_enabled=yes], [ac_newlib_iconv_enabled=no])
+    if test "$ac_newlib_iconv_enabled" = yes; then
+      AC_DEFINE(HAVE_ICONV)
+    fi
+
     AC_DEFINE(HAVE_MEMALIGN)
 
     case "${target}" in
diff --git a/libstdc++-v3/configure b/libstdc++-v3/configure
index fe525308ae28..305675eaa1e1 100755
--- a/libstdc++-v3/configure
+++ b/libstdc++-v3/configure
@@ -28571,7 +28571,31 @@ _ACEOF
 
 
 
-    $as_echo "#define HAVE_ICONV 1" >>confdefs.h
+    # Support for iconv in newlib is configurable.
+    cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+#include <newlib.h>
+int
+main ()
+{
+
+      #ifndef _ICONV_ENABLED
+      #error
+      #endif
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"; then :
+  ac_newlib_iconv_enabled=yes
+else
+  ac_newlib_iconv_enabled=no
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+    if test "$ac_newlib_iconv_enabled" = yes; then
+      $as_echo "#define HAVE_ICONV 1" >>confdefs.h
+
+    fi
 
     $as_echo "#define HAVE_MEMALIGN 1" >>confdefs.h
 
-- 
2.30.2

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