ICU-21107 Update configure files from configure.ac using autoreconf.

icu4c/source/configure

@@ -3515,8 +3515,8 @@ ac_compile='$CC -c $CFLAGS $CPPFLAGS conftest.$ac_ext >&5'
 ac_link='$CC -o conftest$ac_exeext $CFLAGS $CPPFLAGS $LDFLAGS conftest.$ac_ext $LIBS >&5'
 ac_compiler_gnu=$ac_cv_c_compiler_gnu
 
-# Make sure that we try clang++ first, which provides C++11 support.
-# The g++ compiler is less likely to support C++11.
+# Make sure that we try clang++ first, which provides C++17 support.
+# The g++ compiler is less likely to support C++17.
 ac_ext=cpp
 ac_cpp='$CXXCPP $CPPFLAGS'
 ac_compile='$CXX -c $CXXFLAGS $CPPFLAGS conftest.$ac_ext >&5'
@@ -6128,13 +6128,13 @@ $as_echo "no" >&6; }
 fi
 
 if [ "$GXX" = yes ]; then
- # if CXXFLAGS does not have a "-std=" setting, set it now to -std=c++11,
+ # if CXXFLAGS does not have a "-std=" setting, set it now to -std=c++17,
  # and check that the compiler still works.
  if ! echo "$CXXFLAGS" | grep '\-std=' >/dev/null 2>&1; then
  OLD_CXXFLAGS="${CXXFLAGS}"
- CXXFLAGS="$CXXFLAGS -std=c++11"
- { $as_echo "$as_me:${as_lineno-$LINENO}: checking if we have a C++11 compiler" >&5
-$as_echo_n "checking if we have a C++11 compiler... " >&6; }
+ CXXFLAGS="$CXXFLAGS -std=c++17"
+ { $as_echo "$as_me:${as_lineno-$LINENO}: checking if we have a C++17 compiler" >&5
+$as_echo_n "checking if we have a C++17 compiler... " >&6; }
  cat confdefs.h - <<_ACEOF >conftest.$ac_ext
 /* end confdefs.h. */
 
@@ -6147,17 +6147,17 @@ main ()
 }
 _ACEOF
 if ac_fn_cxx_try_compile "$LINENO"; then :
- cxx11_okay=yes
+ cxx17_okay=yes
 else
- cxx11_okay=no
+ cxx17_okay=no
 fi
 rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
- { $as_echo "$as_me:${as_lineno-$LINENO}: result: $cxx11_okay" >&5
-$as_echo "$cxx11_okay" >&6; }
- if [ $cxx11_okay = yes ]; then
- { $as_echo "$as_me:${as_lineno-$LINENO}: Adding CXXFLAGS option -std=c++11" >&5
-$as_echo "$as_me: Adding CXXFLAGS option -std=c++11" >&6;}
- UCONFIG_CXXFLAGS="${UCONFIG_CXXFLAGS} -std=c++11"
+ { $as_echo "$as_me:${as_lineno-$LINENO}: result: $cxx17_okay" >&5
+$as_echo "$cxx17_okay" >&6; }
+ if [ $cxx17_okay = yes ]; then
+ { $as_echo "$as_me:${as_lineno-$LINENO}: Adding CXXFLAGS option -std=c++17" >&5
+$as_echo "$as_me: Adding CXXFLAGS option -std=c++17" >&6;}
+ UCONFIG_CXXFLAGS="${UCONFIG_CXXFLAGS} -std=c++17"
  else
  CXXFLAGS="$OLD_CXXFLAGS"
  fi