Cpp: Set -std=c++11 also if CXXCPP is already set

* m4/ax_cxx_compile_stdxx.m4 (AX_CXX_COMPILE_STDCXX): Set CXXCPP if
neccessary.

--
This fixes the build with scan-build where CXXCPP is already set but
does not include stdc++11. While this deviates from the
autotools-archive version of the script it does not make sense
to me first to check if stdc++11 needs to be set and then not
set it.

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 $switch"
-        fi
         ac_success=yes
         break
       fi
@@ -108,9 +106,7 @@ AC_DEFUN([AX_CXX_COMPILE_STDCXX], [dnl
          CXX="$ac_save_CXX"])
       if eval test x\$$cachevar = xyes; then
         CXX="$CXX $switch"
-        if test -n "$CXXCPP" ; then
         CXXCPP="$CXXCPP $switch"
-        fi
         ac_success=yes
         break
       fi