[3.15] gh-148294: Make configure find g++ correctly (GH-150212)

The `AC_PATH_TOOL` calls had included a duplicated argument, causing a "`PATH`"
consisting of `notfound` to be searched instead of `$PATH`.
(cherry picked from commit c613f72)

Co-authored-by: sendaoYan <yansendao@126.com>

Author: miss-islington

Misc/NEWS.d/next/Build/2026-05-21-15-14-59.gh-issue-148294.VtFaW4.rst

@@ -0,0 +1,2 @@
+Corrected the use of ``AC_PATH_TOOL`` in ``configure.ac`` to allow a C++
+compiler to be found on :envvar:`!PATH`.

configure

@@ -1137,10 +1137,10 @@ preset_cxx="$CXX"
 if test -z "$CXX"
 then
         case "$ac_cv_cc_name" in
-        gcc)    AC_PATH_TOOL([CXX], [g++], [g++], [notfound]) ;;
-        cc)     AC_PATH_TOOL([CXX], [c++], [c++], [notfound]) ;;
-        clang)             AC_PATH_TOOL([CXX], [clang++], [clang++], [notfound]) ;;
-        icc)               AC_PATH_TOOL([CXX], [icpc], [icpc], [notfound]) ;;
+        gcc)    AC_PATH_TOOL([CXX], [g++], [notfound]) ;;
+        cc)     AC_PATH_TOOL([CXX], [c++], [notfound]) ;;
+        clang)             AC_PATH_TOOL([CXX], [clang++], [notfound]) ;;
+        icc)               AC_PATH_TOOL([CXX], [icpc], [notfound]) ;;
         esac
 	if test "$CXX" = "notfound"
 	then