tdf#121131: Default COMPILER_PLUGINS_CXX to g++ -std=c++11

ab8bf553 "Avoid error message from configure
about clang's header for compiler plugins" caused problems after
cd472d1d "Compile as C++2a, where available",
when the compiler used to build LO (CXX) supports -std=gnu++2a but the compiler
used to build loplugin (COMPILER_PLUGINS_CXX) does not.  So solve that problem
instead by just adding -std=c++11 to the default for COMPILER_PLUGINS_CXX.

Change-Id: I95e86bc28f2a0dfb2b7bdce6b15955b870428b8c
Reviewed-on: https://gerrit.libreoffice.org/62997Reviewed-by: Jan-Marek Glogowski <glogow@fbihome.de>
Reviewed-by: Luboš Luňák <l.lunak@collabora.com>
Tested-by: Jenkins

configure.ac

@@ -6723,9 +6723,9 @@ if test "$COM_IS_CLANG" = "TRUE"; then
         save_CPPFLAGS=$CPPFLAGS
         save_CXX=$CXX
         # compiler plugins must be built with "native" compiler that was used to build Clang itself:
-        : "${COMPILER_PLUGINS_CXX=g++}"
+        : "${COMPILER_PLUGINS_CXX=g++ -std=c++11}"
         CXX=$COMPILER_PLUGINS_CXX
-        CPPFLAGS="$CPPFLAGS $CXXFLAGS_CXX11 -I$CLANGDIR/include -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS"
+        CPPFLAGS="$CPPFLAGS -I$CLANGDIR/include -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS"
         AC_CHECK_HEADER(clang/AST/RecursiveASTVisitor.h,
             [COMPILER_PLUGINS=TRUE],
             [