Use clang++ by default.

configure

@@ -2934,7 +2934,7 @@ if test -z "$CXX"; then
     CXX=$CCC
   else
     if test -n "$ac_tool_prefix"; then
-  for ac_prog in g++ c++ gpp aCC CC cxx cc++ cl.exe FCC KCC RCC xlC_r xlC
+  for ac_prog in clang++ g++
   do
     # Extract the first word of "$ac_tool_prefix$ac_prog", so it can be a program name with args.
 set dummy $ac_tool_prefix$ac_prog; ac_word=$2
@@ -2978,7 +2978,7 @@ fi
 fi
 if test -z "$CXX"; then
   ac_ct_CXX=$CXX
-  for ac_prog in g++ c++ gpp aCC CC cxx cc++ cl.exe FCC KCC RCC xlC_r xlC
+  for ac_prog in clang++ g++
 do
   # Extract the first word of "$ac_prog", so it can be a program name with args.
 set dummy $ac_prog; ac_word=$2

configure.ac

@@ -17,7 +17,7 @@ AM_INIT_AUTOMAKE([foreign])
 if test -n "${CXXFLAGS}"; then
   user_set_cxxflags=yes
 fi
-AC_PROG_CXX
+AC_PROG_CXX([clang++ g++])
 AC_ARG_ENABLE(debug, [--enable-debug  Enable debug build (requires debug
  versions of wxWidgets and libstdc++.])
 if test "x${enable_debug}" = "xyes"; then