Makefile.am之C++ 支持

8.9 C++ Support
Automake includes full support for C++.
    Any package including C++ code must define the output variable CXX in ‘configure.ac’;
the simplest way to do this is to use the AC_PROG_CXX macro (see Section “ParticularProgram Checks” in The Autoconf Manual).
    A few additional variables are defined when a C++ source file is seen:

1. CXX，The name of the C++ compiler.
2. CXXFLAGS，Any flags to pass to the C++ compiler.
3. AM_CXXFLAGS，The maintainer’s variant of CXXFLAGS.  CXXFLAGS的变体
4. CXXCOMPILE，The command used to actually compile a C++ source file. The file name is appended to form the complete command line.
5. CXXLINK，The command used to actually link a C++ program.