Allow to override CMAKE_INSTALL_RPATH.

--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -7,7 +7,7 @@
 
 #fix macOS RPATH issues
 set(CMAKE_MACOSX_RPATH 1)
-SET(CMAKE_INSTALL_RPATH "${CMAKE_INSTALL_PREFIX}/lib")
+SET(CMAKE_INSTALL_RPATH "${CMAKE_INSTALL_PREFIX}/lib" CACHE STRING "The rpath to use for installed targets.")
 
 #additional modules for loading libraries
 set (CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${CMAKE_SOURCE_DIR}/cmake/Modules/")