Merge pull request #2 from OpenGeoscience/john/make-postgres-las-optional

John/make postgres las optional
Properly disable tests if BUILD_TESTING is disabled.
Cleanup the linking to liblas to use Usage Requirements.
Install the vtkdatasetreader library and headers.

Also enable fpic if we are building statically, and correct the libraries
install name when building on OSX.
Add in export header support.
Merge pull request #3 from robertmaynard/windows_support

Symbol Exporting and Install Rules
Merge branch 'master' into john/read-feature-properties
Pass Json::Value by reference & const where appropriate