yet another revision of memory limiting: the glibc mallinfo() approach has been shown to be unacceptably broken, so the fallback implementation has been improved

 * mallinfo() is not only broken on 64-bit systems but slows things
   down linearilly to the memory size of the process.
   E.g. after 500000<%A>, SciTECO will act sluggish! Shutting down
   afterwards can take minutes...
   mallinfo() was thus finally discarded as a memory measurement
   technique.
 * Evaluating /proc/self/statm? has also been evaluated and discarded
   because doing this frequently is even slower.
 * Instead, the fallback implementation has been drastically improved:
   * If possible use C++14 global sized deallocators, allowing memory measurements
     across the entire C++ code base with minimal runtime overhead.
     Since we only depend on C++11, a lengthy Autoconf check had to be introduced.
   * Use malloc_usable_size() with global non-sized deallocators to
     measure the approx. memory usage of the entire process (at least
     the ones done via C++).
     The cheaper C++11 sized deallocators implemented via SciTECO::Object still
     have precedence, so this affects Scintilla code only.
 * With both improvements the test case
   sciteco -e '<@EU[X^E\a]"^E\a"%a>'
   is handled sufficiently well now on glibc and performance is much better
   now.
 * The jemalloc-specific technique has been removed since it no longer
   brings any benefits compared to the improved fallback technique.
   Even the case of using malloc_usable_size() in strict C++ mode is
   up to 3 times faster.
 * The new fallback implementation might actually be good enough for
   Windows as well if some MSVCRT-specific support is added, like
   using _msize() instead of malloc_usable_size().
   This must be tested and benchmarked, so we keep the Windows-specific
   implementation for the time being.

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