This will make things as std::to_string() and other C++ features
work with a modern and supported C++ library.
We package up libc++_shared.so and bump the revision of every C++
using package to make it rebuild against it.
Users who have built C++ using libraries or programs will probably
need to rebuild them if they are linked against Termux-supplied
libraries, as user code was previously linked against gnustl while
the system libraries are now using libc++, and it's not a good idea
to mix C++ standard libraries in a program.
Let build-package.sh setup the $PREFIX/lib/libstdc++.so symlink
before building any package, to make sure that all C++ applications
link against a fully featured C++ standard library.
Avoid explicitly declaring a dependency on the libgnustl package
since everyone may link against it at will and instead mark it as
essential.
Assuming that the terminal running mosh in Termux supports 256
colors should be safe, and avoids a popen() call which uses
/system/bin/sh and may not always work.
Set the -l flags ("Locale-related environment variable to try as
part of a fallback environment, if the startup environment does
not specify a character set of UTF-8") when invoking mosh-server
just like the perl script does. Fixes#288.