Complete quoting for parameters of some CMake commands

Some parameters (like "[${PROJECT_SOURCE_DIR}/cmake/modules](https://github.com/PDAL/PDAL/blob/9e8681bea6c783c4ca4539751661dc273d571d0c/CMakeLists.txt#L24)" and "[${PDAL_HEADERS_DIR}/pdal_error.hpp](https://github.com/PDAL/PDAL/blob/84e44c412d776d5c1cc12ac71b27bf7b241aad9a/src/CMakeLists.txt#L26)") are passed to CMake commands in your build scripts without enclosing them by quotation marks. I see that these places will result in build difficulties if the contents of the used variables will contain special characters like spaces.

I would recommend to apply [advices from a Wiki article](http://cmake.org/Wiki/CMake/Language_Syntax#CMake_splits_arguments_unless_you_use_quotation_marks_or_escapes. "CMake splits arguments unless you use quotation marks or escapes.").