Set install_rpath for libcuspatial (#375)

Fix issue where conda installed `libcuspatial` can't find other conda-installed libraries like GDAL.

Related issue in cuDF: rapidsai/cudf#7863

Authors:
  - Paul Taylor (https://github.com/trxcllnt)

Approvers:
  - Ray Douglass (https://github.com/raydouglass)
  - Christopher Harris (https://github.com/cwharris)
  - Keith Kraus (https://github.com/kkraus14)

URL: #375

cpp/CMakeLists.txt

@@ -139,6 +139,7 @@ add_library(cuspatial SHARED
 
 set_target_properties(cuspatial
     PROPERTIES BUILD_RPATH                         "\$ORIGIN"
+               INSTALL_RPATH                       "\$ORIGIN"
                # set target compile options
                CXX_STANDARD                        14
                CXX_STANDARD_REQUIRED               ON