build: update ax_cxx_compile_stdcxx to serial 4

build-aux/m4/ax_cxx_compile_stdcxx.m4

@@ -9,9 +9,9 @@
 # DESCRIPTION
 #
 #   Check for baseline language coverage in the compiler for the specified
-#   version of the C++ standard.  If necessary, add switches to CXXFLAGS to
-#   enable support.  VERSION may be '11' (for the C++11 standard) or '14'
-#   (for the C++14 standard).
+#   version of the C++ standard.  If necessary, add switches to CXX and
+#   CXXCPP to enable support.  VERSION may be '11' (for the C++11 standard)
+#   or '14' (for the C++14 standard).
 #
 #   The second argument, if specified, indicates whether you insist on an
 #   extended mode (e.g. -std=gnu++11) or a strict conformance mode (e.g.
@@ -39,7 +39,7 @@
 #   and this notice are preserved.  This file is offered as-is, without any
 #   warranty.
 
-#serial 1
+#serial 4
 
 dnl  This macro is based on the code from the AX_CXX_COMPILE_STDCXX_11 macro
 dnl  (serial version number 13).
@@ -74,14 +74,17 @@ AC_DEFUN([AX_CXX_COMPILE_STDCXX], [dnl
       cachevar=AS_TR_SH([ax_cv_cxx_compile_cxx$1_$switch])
       AC_CACHE_CHECK(whether $CXX supports C++$1 features with $switch,
                      $cachevar,
-        [ac_save_CXXFLAGS="$CXXFLAGS"
-         CXXFLAGS="$CXXFLAGS $switch"
+        [ac_save_CXX="$CXX"
+         CXX="$CXX $switch"
          AC_COMPILE_IFELSE([AC_LANG_SOURCE([_AX_CXX_COMPILE_STDCXX_testbody_$1])],
           [eval $cachevar=yes],
           [eval $cachevar=no])
-         CXXFLAGS="$ac_save_CXXFLAGS"])
+         CXX="$ac_save_CXX"])
       if eval test x\$$cachevar = xyes; then
-        CXXFLAGS="$CXXFLAGS $switch"
+        CXX="$CXX $switch"
+        if test -n "$CXXCPP" ; then
+          CXXCPP="$CXXCPP $switch"
+        fi
         ac_success=yes
         break
       fi
@@ -97,14 +100,17 @@ AC_DEFUN([AX_CXX_COMPILE_STDCXX], [dnl
       cachevar=AS_TR_SH([ax_cv_cxx_compile_cxx$1_$switch])
       AC_CACHE_CHECK(whether $CXX supports C++$1 features with $switch,
                      $cachevar,
-        [ac_save_CXXFLAGS="$CXXFLAGS"
-         CXXFLAGS="$CXXFLAGS $switch"
+        [ac_save_CXX="$CXX"
+         CXX="$CXX $switch"
          AC_COMPILE_IFELSE([AC_LANG_SOURCE([_AX_CXX_COMPILE_STDCXX_testbody_$1])],
           [eval $cachevar=yes],
           [eval $cachevar=no])
-         CXXFLAGS="$ac_save_CXXFLAGS"])
+         CXX="$ac_save_CXX"])
       if eval test x\$$cachevar = xyes; then
-        CXXFLAGS="$CXXFLAGS $switch"
+        CXX="$CXX $switch"
+        if test -n "$CXXCPP" ; then
+          CXXCPP="$CXXCPP $switch"
+        fi
         ac_success=yes
         break
       fi
@@ -115,7 +121,7 @@ AC_DEFUN([AX_CXX_COMPILE_STDCXX], [dnl
     if test x$ac_success = xno; then
       AC_MSG_ERROR([*** A compiler with support for C++$1 language features is required.])
     fi
-  else
+  fi
   if test x$ac_success = xno; then
     HAVE_CXX$1=0
     AC_MSG_NOTICE([No compiler with C++$1 support was found])
@@ -124,9 +130,7 @@ AC_DEFUN([AX_CXX_COMPILE_STDCXX], [dnl
     AC_DEFINE(HAVE_CXX$1,1,
               [define if the compiler supports basic C++$1 syntax])
   fi
-
   AC_SUBST(HAVE_CXX$1)
-  fi
 ])