Decouple setting -std=gnu++11/c++11 from setting -stdlib=libc++

Forcing -stdlib=c++11 into CXX upon --enable-libc++ instead of only into
CXXFLAGS_CXX11 as done normally caused external projects to unexpectedly pick it
up, causing build failures in firebird.

Change-Id: I588d75ccbde6b9fd4f38e10a0c67c2f6086f8977

configure.ac

@@ -2690,8 +2690,7 @@ if test $_os = Darwin; then
                     AC_MSG_ERROR([--enable-libc++ requires --with-macosx-version-min-required >= 10.7])
                 fi
                 # Use libc++ instead of libstdc++ when possible
-                # and also compile as C++11
-                stdlib="-std=c++11 -stdlib=libc++"
+                stdlib=-stdlib=libc++
                 CPP_LIBRARY="LIBCPP"
             fi
             if test "$ENABLE_LTO" = TRUE; then
@@ -5956,11 +5955,6 @@ if test "$COM" = MSC; then
 elif test "$GCC" = "yes"; then
     CXXFLAGS_CXX11=
     AC_MSG_CHECKING([whether $CXX supports C++11])
-    if test "$CPP_LIBRARY" = LIBCPP -a $_os = Darwin; then
-        : Already set CXX to contain -std=c++11
-        HAVE_CXX11=TRUE
-        AC_MSG_RESULT(yes)
-    else
     for flag in -std=gnu++11 -std=gnu++0x -std=c++11 -std=c++0x ; do
         save_CXXFLAGS=$CXXFLAGS
         CXXFLAGS="$CXXFLAGS $flag -Werror"
@@ -5978,7 +5972,6 @@ elif test "$GCC" = "yes"; then
     else
         AC_MSG_RESULT(no)
     fi
-    fi
 fi
 
 if test "$HAVE_CXX11" = TRUE; then