Unable to compile Tutorial example on Linux platforms

boost version : 1.72

I have some troubles compiling the following program on Linux platforms (Raspberry Pi / Linux Debian 10)

```
#include
#include

using namespace boost::process;

int main()
{
ipstream pipe_stream;
boost::process::child c(boost::process::search_path("g++"));

std::string line;

while (pipe_stream && std::getline(pipe_stream, line) && !line.empty())
std::cerr << line << std::endl;

c.wait();
}
```

The error is the following:
```
/home/pi/Travail/ThirdParties/boost_1_72_0/boost/fusion/container/vector/detail/cpp03/preprocessed/vector10.hpp:41:22: error: use of deleted function ‘boost::process::detail::posix::exe_cmd_init::exe_cmd_init(const boost::process::detail::posix::exe_cmd_init&)’
: m0(arg0) {}
^
In file included from /home/pi/Travail/ThirdParties/boost_1_72_0/boost/process/detail/basic_cmd.hpp:20:0,
from /home/pi/Travail/ThirdParties/boost_1_72_0/boost/process/args.hpp:33,
from /home/pi/Travail/ThirdParties/boost_1_72_0/boost/process.hpp:22,
from ../src/main.cpp:1:
/home/pi/Travail/ThirdParties/boost_1_72_0/boost/process/detail/posix/basic_cmd.hpp:106:5: note: declared here
exe_cmd_init(const exe_cmd_init & ) = delete;
^~~~~~~~~~~~
src/subdir.mk:18: recipe for target 'src/main.o' failed
make: *** [src/main.o] Error 1
```

It seems that changing exec_cmd_init default constructor declaration from` exe_cmd_init(const exe_cmd_init & ) = delete; `to `exe_cmd_init(const exe_cmd_init & ) = default;` (line 106 of file /boost/process/detail/posix/basic_cmd.hpp) fixes the compilation issue.