Fix various compiler warnings

* Mark boost and python headers as system headers to avoid a pile of
  (mostly boost) warnings in headers we don't control.

* Remove assertion about the return value of memcpy, because memcpy
  unconditionally returns the dest buffer regardless of success.

* Avoid unused variable warnings
  - When building without hexer
  - Due to variables only used in asserts
  - Unused endianess variable

* Fix bug in las writer when reading uint8_t intensity from buffer,
  which would have accessed a bogus pointer and caused a segfault.