cmake: make sure that lib's are lib and not lib64 for musrfit.

CMakeLists.txt

@@ -12,6 +12,7 @@ set(CMAKE_CXX_STANDARD 17)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 set(CMAKE_CXX_EXTENSIONS OFF)
 
+set(CMAKE_INSTALL_LIBDIR lib CACHE PATH "Object code libraries")
 include(GNUInstallDirs)
 
 #--- musrfit specific options -------------------------------------------------
@@ -280,16 +281,16 @@ set(CMAKE_SKIP_BUILD_RPATH FALSE)
 # (but later on when installing)
 set(CMAKE_BUILD_WITH_INSTALL_RPATH FALSE)
 
-set(CMAKE_INSTALL_RPATH "${CMAKE_INSTALL_PREFIX}/lib")
+set(CMAKE_INSTALL_RPATH "${CMAKE_INSTALL_FULL_LIBDIR}")
 
 # add the automatically determined parts of the RPATH
 # which point to directories outside the build tree to the install RPATH
 set(CMAKE_INSTALL_RPATH_USE_LINK_PATH TRUE)
 
 # the RPATH to be used when installing, but only if it's not a system directory
-list(FIND CMAKE_PLATFORM_IMPLICIT_LINK_DIRECTORIES "${CMAKE_INSTALL_PREFIX}/lib" isSystemDir)
+list(FIND CMAKE_PLATFORM_IMPLICIT_LINK_DIRECTORIES "${CMAKE_INSTALL_FULL_LIBDIR}" isSystemDir)
 if("${isSystemDir}" STREQUAL "-1")
-    set(CMAKE_INSTALL_RPATH "${CMAKE_INSTALL_PREFIX}/lib")
+    set(CMAKE_INSTALL_RPATH "${CMAKE_INSTALL_FULL_LIBDIR}")
 endif("${isSystemDir}" STREQUAL "-1")
 if (UNIX)
   set(rpath ${CMAKE_INSTALL_RPATH})