Run AC_PROG_CXX in the right place.

configure

@@ -10068,9 +10068,12 @@ fi
 
 # The C++ compiler, if desired.
 
-# The following test is quick and harmless.
-# It defaults to 'g++' if no compiler is found.
-ac_ext=cpp
+# Now actually check that the C++ compiler works if it is needed.
+want_cxx=no
+if test $enable_cxx != no; then
+  test_CXXFLAGS=${CXXFLAGS+set}
+
+  ac_ext=cpp
 ac_cpp='$CXXCPP $CPPFLAGS'
 ac_compile='$CXX -c $CXXFLAGS $CPPFLAGS conftest.$ac_ext >&5'
 ac_link='$CXX -o conftest$ac_exeext $CXXFLAGS $CPPFLAGS $LDFLAGS conftest.$ac_ext $LIBS >&5'
@@ -10328,11 +10331,6 @@ ac_link='$CC -o conftest$ac_exeext $CFLAGS $CPPFLAGS $LDFLAGS conftest.$ac_ext $
 ac_compiler_gnu=$ac_cv_c_compiler_gnu
 
 
-# Now actually check that the C++ compiler works if it is needed.
-want_cxx=no
-if test $enable_cxx != no; then
-  test_CXXFLAGS=${CXXFLAGS+set}
-
   echo "CXXFLAGS chosen by autoconf: $CXXFLAGS" >&5
   cxxflags_ac_prog_cxx=$CXXFLAGS
   cxxflags_list=ac_prog_cxx

configure.ac

@@ -1753,15 +1753,13 @@ AC_SUBST(MPIR_AS)
 
 # The C++ compiler, if desired.
 
-# The following test is quick and harmless.
-# It defaults to 'g++' if no compiler is found.
-AC_PROG_CXX
-
 # Now actually check that the C++ compiler works if it is needed.
 want_cxx=no
 if test $enable_cxx != no; then
   test_CXXFLAGS=${CXXFLAGS+set}
 
+  AC_PROG_CXX
+
   echo "CXXFLAGS chosen by autoconf: $CXXFLAGS" >&AC_FD_CC
   cxxflags_ac_prog_cxx=$CXXFLAGS
   cxxflags_list=ac_prog_cxx