erts: Use AC_PROG_CXX to find C++ compiler
erts: Use AC_PROG_CXX to find C++ compiler

We previously used AC_CHECK_TOOLS() with a list of C++ compiler names
to find a C++ compiler. As of autconf 2.71 AC_CHECK_TOOLS() detects that
we try to find a C++ compiler and change mode to C++. It, however, fails
to restore to C mode when done and instead "restore" to C++ mode. This
causes the tests that follow to execute in C++ mode when C mode is
expected.

AC_PROG_CXX push and pop languages as expected. It also by default check
all C++ compiler names that we had in the list.

Since it works using autoconf 2.69, we only change this when using
autoconf 2.71.

erlang/otp,jj1bdx/otp,isvilen/otp,dumbbell/otp,erlang/otp,dumbbell/otp,jj1bdx/otp,erlang/otp,potatosalad/otp,dumbbell/otp,dgud/otp,mikpe/otp,isvilen/otp,mikpe/otp,dgud/otp,getong/otp,vinoski/otp,bjorng/otp,jj1bdx/otp,dgud/otp,erlang/otp,erlang/otp,mikpe/otp,rlipscombe/otp,isvilen/otp,emacsmirror/erlang,jj1bdx/otp,emacsmirror/erlang,vinoski/otp,vinoski/otp,emacsmirror/erlang,jj1bdx/otp,emacsmirror/erlang,vinoski/otp,rlipscombe/otp,emacsmirror/erlang,erlang/otp,g-andrade/otp,jj1bdx/otp,dumbbell/otp,getong/otp,vinoski/otp,dgud/otp,potatosalad/otp,potatosalad/otp,getong/otp,mikpe/otp,rlipscombe/otp,getong/otp,emacsmirror/erlang,dgud/otp,rlipscombe/otp,isvilen/otp,bjorng/otp,rlipscombe/otp,g-andrade/otp,vinoski/otp,g-andrade/otp,emacsmirror/erlang,potatosalad/otp,g-andrade/otp,potatosalad/otp,bjorng/otp,bjorng/otp,rlipscombe/otp,bjorng/otp,erlang/otp,dgud/otp,getong/otp,getong/otp,mikpe/otp,mikpe/otp,dumbbell/otp,mikpe/otp,getong/otp,dgud/otp,dumbbell/otp,getong/otp,isvilen/otp,isvilen/otp,g-andrade/otp,bjorng/otp,bjorng/otp,getong/otp,rlipscombe/otp,getong/otp,g-andrade/otp,potatosalad/otp,jj1bdx/otp,dgud/otp,isvilen/otp,mikpe/otp,rlipscombe/otp,isvilen/otp,potatosalad/otp,dumbbell/otp,dgud/otp,dumbbell/otp,dumbbell/otp,jj1bdx/otp,dgud/otp,jj1bdx/otp,g-andrade/otp,mikpe/otp,vinoski/otp,potatosalad/otp,dumbbell/otp,bjorng/otp,erlang/otp,jj1bdx/otp,vinoski/otp,bjorng/otp,rlipscombe/otp,bjorng/otp,erlang/otp,g-andrade/otp,isvilen/otp,potatosalad/otp,emacsmirror/erlang,g-andrade/otp,rlipscombe/otp,erlang/otp,g-andrade/otp,emacsmirror/erlang,potatosalad/otp,mikpe/otp,emacsmirror/erlang,vinoski/otp,vinoski/otp,isvilen/otp