Set CMAKE_INSTALL_RPATH for non-system install prefix

During install set RPATH so that libtbb can be found when building
with the oneTBB submodule.
The following programs in the tools directory require libtbb:

- compress_inverted_index
- evaluate_queries
- invert
- parse_collection
- partition_fwd_index
- reorder-docids
- shards

See #242

CMakeLists.txt

@@ -85,6 +85,12 @@ if (UNIX)
      set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -ggdb") # Add debug info anyway
    endif()
 
+   # modify RPATH when installing to a non-system directory (e.g. /usr/local).
+   # required to find libtbb with non-system libtbb.
+   list(FIND CMAKE_PLATFORM_IMPLICIT_LINK_DIRECTORIES "${CMAKE_INSTALL_PREFIX}/lib" isSystemDir)
+   if(NOT PISA_SYSTEM_ONETBB AND "${isSystemDir}" STREQUAL "-1")
+      set(CMAKE_INSTALL_RPATH "${CMAKE_INSTALL_PREFIX}/lib")
+   endif()
 endif()
 
 find_package(OpenMP)