configure.ac: Display C++ compiler options

C++ compiler options had not been shown at the end of the run of the
configure script.  Now show them.

configure.ac

@@ -669,7 +669,8 @@ echo \
  ${PACKAGE_NAME} Version ${PACKAGE_VERSION} configuration:
 
  Prefix         ${prefix}
- Compiler       ${CC} ${CFLAGS} ${CPPFLAGS}
+ C Compiler     ${CC} ${CFLAGS} ${CPPFLAGS}
+ C++ Compiler   ${CXX} ${CXXFLAGS} ${CPPFLAGS}
 
 
  Package features: