There are 3 ways that calls directly to standard allocator functions can
be dealt with:
1. Block their use, cause them to generate an error.  This is the default.
2. Replace them with the Asterisk equivalent function calls.
3. Leave them alone.

This change allows one of these 3 options to be selected by any source.
The source just needs to define ASTMM_LIBC to ASTMM_BLOCK, ASTMM_REDIRECT,
or ASTMM_IGNORE to use option 1, 2 or 3 respectively.  Normally ASTMM_BLOCK
is the correct option, so it is default when ASTMM_LIBC is not defined.
In some cases when building 3rd party code it is desirable to have it use
Asterisk functions, without changing the whole source - ASTMM_REDIRECT
accomplishes this.  When using 3rd party libraries sometimes a static
inline function will make use of malloc or free.  In these cases it may
be unsafe to replace the allocator in the header, as it's possible the
memory could be freed by the library using standard allocators.  For
those cases ASTMM_IGNORE is needed.

Change-Id: I8afef4bc7f3b93914263ae27d3a5858b69663fc7