Replace deprecated command make_directory

It was marked as deprecated by CMake 2.6 or even earlier.
Build with FLANN support
Build with PostgreSQL Point Cloud support
Update FindPostgreSQL.cmake definition

Improved version from https://github.com/mloskot/cmake-modules
Checking if it solves issue with finding PostgreSQL libraries for pgpointcloud,
previous version of the module failed (Travis CI build 20).
Always notify #pdal on successful build
Fix segfault when reading from las into multiple buffers

In the presence of multiple buffers (such as the temporary buffers used
by the Mosaic stage) caching the dimensions when reading las doesn't
work: dimensions may be in different locations in different buffers.
The previously used buffers may even be deallocated, in which case the
cached dimensions will be stale pointers, leading to segfaults.

The fix here simply removes dimension caching entirely.  With any
reasonable chunk size this shouldn't lead to performance degradation.
Allow SRS to be ignored in Mosaic filter

Ignoring the SRS is useful when pdal has been compiled without gdal
support, and the user knows the inputs have a shared spatial reference
system.  (Attempting SRS comparison in this case would otherwise result
in an exception.)
Merge pull request #147 from c42f/mosaic_srs_ignore

Allow SRS to be ignored in Mosaic filter
Merge pull request #146 from c42f/las_reader_cache_dim_fix

Fix segfault when reading from las into multiple buffers
Fix minor memory leak of stage factory