Revert "Use relative rpath so that the installation and build dirs are relocatable."

This reverts commit r201921.

This should bring the polly bots back. I will try to build it locally to
understand how cmake was setting the rpath of LLVMPolly.so.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201934 91177308-0d34-0410-b5e6-96231b3b80d8

CMakeLists.txt

@@ -423,13 +423,9 @@ set( CMAKE_RUNTIME_OUTPUT_DIRECTORY ${LLVM_BINARY_DIR}/bin )
 set( CMAKE_LIBRARY_OUTPUT_DIRECTORY ${LLVM_BINARY_DIR}/lib )
 set( CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${LLVM_BINARY_DIR}/lib )
 
-set(CMAKE_BUILD_WITH_INSTALL_RPATH ON)
-if (APPLE)
-  set(CMAKE_INSTALL_NAME_DIR "@rpath")
-  set(CMAKE_INSTALL_RPATH "@executable_path/../lib")
-else(UNIX)
-  set(CMAKE_INSTALL_RPATH "\$ORIGIN/../lib")
-endif()
+if( NOT DEFINED CMAKE_INSTALL_RPATH )
+  set( CMAKE_INSTALL_RPATH "\$ORIGIN/../lib")
+endif( NOT DEFINED CMAKE_INSTALL_RPATH )
 
 set(CMAKE_INCLUDE_CURRENT_DIR ON)