Log Message:
libspectrum 1.3.6:

* Fix Z80 snapshot writing when +D is enabled
* Export pkgconfig file to publish library version
* Fix offset of keyboard mappings in Z80 v3 snaphots
* Various minor bug fixes/improvements:
  * Replace '*' in boolean context with '&&' in libspectrum_malloc0_n()
  * Fix warning about missing atomic_lock()/atomic_unlock() declarations
    in gslock.c