par2tbb fails to compile with an "auto_ptr is not a member of std" error

I am trying to compile par2tbb on Gentoo Linux kernel 3.18.12, and I keep getting the following compilation error:

par2cmdline.cpp: In function ‘int main(int, char**)’:
par2cmdline.cpp:88:3: error: ‘auto_ptr’ is not a member of ‘std’
   std::auto_ptr<CommandLine> commandline(new CommandLine);
   ^

I already checked that #include <memory.h> was present on the source code, I already added CXXFLAGS = -std=c++11 to the makefile, and it still won't compile.

Looking up online, these two measures are the only fixes I could find. Any idea on what's going on?