diff --git a/m4/ax_cxx_compile_stdcxx.m4 b/m4/ax_cxx_compile_stdcxx.m4
index 2c18e49c..de3d0121 100644
--- a/m4/ax_cxx_compile_stdcxx.m4
+++ b/m4/ax_cxx_compile_stdcxx.m4
@@ -82,9 +82,7 @@ AC_DEFUN([AX_CXX_COMPILE_STDCXX], [dnl
          CXX="$ac_save_CXX"])
       if eval test x\$$cachevar = xyes; then
         CXX="$CXX $switch"
-        if test -n "$CXXCPP" ; then
-          CXXCPP="$CXXCPP $switch"
-        fi
+        CXXCPP="$CXXCPP $switch"
         ac_success=yes
         break
       fi
@@ -108,9 +106,7 @@ AC_DEFUN([AX_CXX_COMPILE_STDCXX], [dnl
          CXX="$ac_save_CXX"])
       if eval test x\$$cachevar = xyes; then
         CXX="$CXX $switch"
-        if test -n "$CXXCPP" ; then
-          CXXCPP="$CXXCPP $switch"
-        fi
+        CXXCPP="$CXXCPP $switch"
         ac_success=yes
         break
       fi