configure: Add --enable-static-LIBRARY options for selective static linking

Statically linking mosh-server with glibc isn’t a great idea for
various reasons (nsswitch modules, locale format incompatibilities).
But we can provide most of the benefits of static linking by allowing
specific library dependencies to be linked statically using -Bstatic
and -Bdynamic.  The full set is enabled by

    ./configure --enable-static-libraries

which is equivalent to

    ./configure --enable-static-libstdc++ --enable-static-libgcc \
      --enable-static-utempter --enable-static-zlib --enable-static-curses \
      --enable-static-crypto --enable-static-protobuf

and results in binaries whose only runtime library dependencies are
provided with libc:

    $ ldd src/frontend/mosh-server
            linux-vdso.so.1 (0x00007ffe0b377000)
            libutil.so.1 => /lib/x86_64-linux-gnu/libutil.so.1 (0x00007fa0d9970000)
            libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007fa0d97e3000)
            libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007fa0d97c2000)
            libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007fa0d95d8000)
            /lib64/ld-linux-x86-64.so.2 (0x00007fa0d9f6a000)

Signed-off-by: Anders Kaseorg <andersk@mit.edu>