Since the introduction of the STL wrappers, they have included
mozalloc.h, and multiple times, we've hit header reentrancy problems,
and worked around them as best as we could.
Taking a step back, all mozalloc.h does is:
- declare moz_* allocator functions.
- define inline implementations of various operator new/delete variants.
The first only requires the functions to be declared before they are used,
so mozalloc.h only needs to be included before anything that would use
those functions.
The second doesn't actually require a specific order, as long as the
declaration for those functions comes before their use, and they are
either declared in <new> or implicitly by the C++ compiler.
So all in all, it doesn't matter that mozalloc.h is included before the
wrapped STL headers. What matters is that it's included when STL headers
are included. So arrange things such that mozalloc.h is included after
the first wrapped STL header is fully preprocessed (and all its includes
have been included).
It turns out that, since we're including <new> before setting
_GLIBCXX_DEBUG, the debug parts of c++config.h are not activated, and
that has an impact of how much of the debug features of the STL are
activated.
In contrast, the upcoming changes to the STL wrappers are avoiding the
include of <new> before _GLIBCXX_DEBUG is set, which in turn breaks the
build because, as we link things that use STL wrappers with things that
don't, they end up with a different state of STL debugging, and have
mismatching symbols.
Our STL wrappers do various different things, one of which is including
mozalloc.h for infallible operator new. mozalloc.h includes stdlib.h,
which, in libstdc++ >= 6 is now itself a wrapper around cstdlib, which
circles back to our STL wrapper.
But of the things our STL wrappers do, including mozalloc.h is not one
that is necessary for cstdlib. So skip including mozalloc.h in our
cstdlib wrapper.
Additionally, some C++ sources (in media/mtransport) are including
headers in an extern "C" block, which end up including stdlib.h, which
ends up including cstdlib because really, this is all C++, and our
wrapper pre-includes <new> for mozalloc.h, which fails because templates
don't work inside extern "C". So, don't pre-include <new> when we're not
including mozalloc.h.
