Fix compilation of build/*.o when using C++.

Fix compilation of build/*.o when using C++.

Found this while testing the C++ conversion for vec.[ch] on the
cxx-conversion branch.  We do not build the build/*.o files with g++,
so I was getting lots of syntax errors while compiling build/vec.o.

2012-05-31   Diego Novillo  <dnovillo@google.com>

       * configure.ac (CXX_FOR_BUILD): Define and substitute.
       (BUILD_CXXFLAGS): Define and substitute.
       * Makefile.in (BUILD_CXXFLAGS): Define.
       (CXX_FOR_BUILD): Define.
       (COMPILER_FOR_BUILD): Set to CXX_FOR_BUILD if building with C++.
       (LINKER_FOR_BUILD): Likewise.
       (BUILD_COMPILERFLAGS): Set to BUILD_CXXFLAGS if building with C++.
       (BUILD_LINKERFLAGS): Likewise.
       * configure: Regenerate.

From-SVN: r188069