diff --git a/.travis.yml b/.travis.yml
index 4db9f00f25..eeba2d2305 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -884,7 +884,7 @@ jobs:
 #OK#  - env: NUT_MATRIX_TAG="gnu99-gcc-7-nowarn" BUILD_TYPE=default-all-errors CFLAGS="-std=gnu99" CXXFLAGS="-std=gnu++99" CC=gcc-7 CXX=g++-7
 #OK#  - env: NUT_MATRIX_TAG="gnu11-gcc-7-nowarn" BUILD_TYPE=default-all-errors CFLAGS="-std=gnu11" CXXFLAGS="-std=gnu++11" CC=gcc-7 CXX=g++-7
 #OK#  - env: NUT_MATRIX_TAG="gnu17-gcc-9-nowarn" BUILD_TYPE=default-all-errors CFLAGS="-std=gnu17" CXXFLAGS="-std=gnu++17" CC=gcc-9 CXX=g++-9
-  - env: NUT_MATRIX_TAG="gnu99-clang-5.0-nowarn" BUILD_TYPE=default-all-errors CFLAGS="-std=gnu99" CXXFLAGS="-std=gnu++99" CC=clang-5.0 CXX=clang++-5.0
+#OK#  - env: NUT_MATRIX_TAG="gnu99-clang-5.0-nowarn" BUILD_TYPE=default-all-errors CFLAGS="-std=gnu99" CXXFLAGS="-std=gnu++99" CC=clang-5.0 CXX=clang++-5.0
 #OK#  - env: NUT_MATRIX_TAG="gnu17-clang-8-nowarn" BUILD_TYPE=default-all-errors CFLAGS="-std=gnu17" CXXFLAGS="-std=gnu++17" CC=clang-8 CXX=clang++-8
 #OK#  - env: NUT_MATRIX_TAG="gnu17-gcc-9-warn" BUILD_TYPE=default-all-errors CFLAGS="-Wall -Wextra -Werror -pedantic -std=gnu17" CXXFLAGS="-Wall -Wextra -Werror -std=gnu++17" CC=gcc-9 CXX=g++-9
   - env: NUT_MATRIX_TAG="c99-clang-3.5-nowarn" BUILD_TYPE=default-all-errors CFLAGS="-std=c99" CXXFLAGS="-std=c++99" CC=clang-3.5 CXX=clang++-3.5
diff --git a/configure.ac b/configure.ac
index 07e7b2d8a1..a3609c5d75 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1467,17 +1467,22 @@ CPLUSPLUS_MAIN='printf("%ld\n", __cplusplus);'
 AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[${CPLUSPLUS_DECL}]], [[${CPLUSPLUS_MAIN}]])],
     [AC_MSG_RESULT([yes, out of the box])
      have_cxx11=yes],
-    [CXXFLAGS="$CXXFLAGS -std=c++11"
-     AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[${CPLUSPLUS_DECL}]], [[${CPLUSPLUS_MAIN}]])],
-        [AC_MSG_RESULT([yes, GCC-style (as C++11)])
-         have_cxx11=yes],
-        [CXXFLAGS="$CXXFLAGS -std=c++0x"
-         AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[${CPLUSPLUS_DECL}]], [[${CPLUSPLUS_MAIN}]])],
-            [AC_MSG_RESULT([yes, GCC-style (as C++0X)])
-             have_cxx11=yes],
-            [AC_MSG_RESULT([no])
-             CXXFLAGS="$my_CXXFLAGS"
-             have_cxx11=no])])])
+    [AS_CASE(["${CXXFLAGS}"],
+        [*"-std="*], [
+            AC_MSG_RESULT([no, not with the standard already set in CXXFLAGS='${CXXFLAGS}'])
+            have_cxx11=no,
+        ],[
+            CXXFLAGS="$CXXFLAGS -std=c++11"
+            AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[${CPLUSPLUS_DECL}]], [[${CPLUSPLUS_MAIN}]])],
+                [AC_MSG_RESULT([yes, GCC-style (as C++11)])
+                 have_cxx11=yes],
+                [CXXFLAGS="$CXXFLAGS -std=c++0x"
+                 AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[${CPLUSPLUS_DECL}]], [[${CPLUSPLUS_MAIN}]])],
+                    [AC_MSG_RESULT([yes, GCC-style (as C++0X)])
+                     have_cxx11=yes],
+                    [AC_MSG_RESULT([no])
+                     CXXFLAGS="$my_CXXFLAGS"
+                     have_cxx11=no])])])])
 AM_CONDITIONAL(HAVE_CXX11, test "${have_cxx11}" = "yes")
 AC_LANG_POP([C++])
 unset CPLUSPLUS_MAIN