Since we switched from autotools to Meson in commit
94592c14, we don't need to include
`config.h` early to properly enable large file support.  Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.

This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.

We can remove those C++11 and C++14 kludges because we require GCC 4.9
now.

The "packed" attribute triggers a clang 4.0 warning, and it's not
necessary.  All we want is correct allocation of this
dynamically-sized struct.

The "packed" attribute triggers a clang 4.0 warning, and it's not
necessary.  All we want is correct allocation of this
dynamically-sized struct.

When a reference counter is at its limit, don't allocate a new
TagPoolSlot - that would result in many TagPoolSlot instances with
ref==1.  This in turn would make the linked list very very large,
which means quadratic runtime for many operations.

By accident, this declared a global variable instead of adding the
"packed" attribute.

Eliminates explicit global initialisation.

After we've been hit by Large File Support problems several times in
the past week (which only occur on 32 bit platforms, which I don't
have), this is yet another attempt to fix the issue.

The strings passed to tag_pool_get_item() are not null-terminated, and
the caller passes the string length.  Don't assume it is
null-terminated anyway by using strcmp().