[pdal] first build notes: boost namespace, missing include, laszip include guards

[Bradley Chambers]

I've noted a number of updates recently to make better use of the
boost:: namespace when defining uint8_t, uint16_t, etc. These changes
do not appear to have made it to
src/drivers/las/VariableLengthRecord.cpp quite yet. Along the same
lines, it's probably not a bad idea to specifically add #include
<boost/cstdint.hpp> in any of the files where these types are being
used (although I think in many cases it is being included through
other header files).

I also found that I needed to add #include <pdal/exceptions.hpp> to
src/drivers/las/LasHeaderWriter.cpp.

If laszip is optional, would everyone agree that all associated
includes should be guarded with PDAL_HAVE_LASZIP? I've found
references in the following files:
- src/drivers/las/Iterator.cpp
- src/drivers/las/Reader.cpp

Brad