Use rpaths for linked libraries

.cmake/isce2_buildflags.cmake

@@ -14,13 +14,15 @@ set(CMAKE_CXX_STANDARD 11)
 set(CMAKE_CXX_STANDARD_REQUIRED y)
 set(CMAKE_CXX_EXTENSIONS n)
 
-# TODO (fix RPATHs)
-# We have to hack our RPATHs a bit for these shared libraries to be
-# loaded by others on the install-side. Maybe these libraries should
-# be combined and/or installed to a common ISCE2 lib directory.
-# Is there a semantic way to propagate their RPATHs
-# without using these global variables?
+include(GNUInstallDirs)
+
+# add automatically determined parts of the RPATH, which point to directories
+# outside of the build tree, to the install RPATH
 set(CMAKE_INSTALL_RPATH_USE_LINK_PATH ON)
-list(APPEND CMAKE_INSTALL_RPATH
-    ${CMAKE_INSTALL_PREFIX}/${ISCE2_PKG}/components/isceobj/Util
-    )
+
+# the RPATH to be used when installing, but only if it's not a system directory
+set(abs_libdir ${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_LIBDIR})
+list(FIND CMAKE_PLATFORM_IMPLICIT_LINK_DIRECTORIES ${abs_libdir} isSystemDir)
+if("${isSystemDir}" STREQUAL "-1")
+    list(APPEND CMAKE_INSTALL_RPATH ${abs_libdir})
+endif()

components/isceobj/Util/CMakeLists.txt

@@ -130,7 +130,8 @@ target_include_directories(combinedLib INTERFACE
 install(TARGETS
     utilLib
     combinedLib
-    LIBRARY DESTINATION lib)
+    LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}
+    )
 
 InstallSameDir(
     combinedlibmodule