added ./configure --enable-debug and make sure that NDEBUG is defined properly

* This simplifies writing CFLAGS="-g -O0" CXXFLAGS="-g -O0".
* We build "release" binaries by default.
  NDEBUG will now be defined unless you specify --enable-debug.
  This enables some optimizations that have long been implemented but were never actually active:
  * SciTECO shuts down faster since it will not explicitly free memory.
    On the downside, this would complicate memory debugging with Valgrind/memcheck.
  * dlmalloc is built with -DINSECURE=1 which is supposedly a bit faster.
    Some compilers also complained about an unportable preprocessor usage which should
    now be gone.
* All CI builds are now with --enable-debug.
  This will slow them down but ensure that more code is executed and thus tested.

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