LIBDIR_SUFFIX is only used in one location, while LIB_SUFFIX is set already.

--- a/config/vigra-config.in	2017-05-19 17:01:08.000000000 +0200
+++ b/config/vigra-config.in	2019-04-08 22:33:18.938060195 +0200
@@ -68,7 +68,7 @@
 if op.impex_lib: # was: --impex_lib|--impex-lib|--libs
     ldflags = []
 
-    libDir = '@CMAKE_INSTALL_PREFIX@/lib@LIBDIR_SUFFIX@'
+    libDir = '@CMAKE_INSTALL_PREFIX@/lib@LIB_SUFFIX@'
     if libDir not in standardLibDirs:
         ldflags.append('-L' + libDir)
 
--- a/config/VigraSetDefaults.cmake	2017-05-19 17:01:08.000000000 +0200
+++ b/config/VigraSetDefaults.cmake	2019-04-08 22:33:09.361974643 +0200
@@ -59,13 +59,6 @@
     CACHE BOOL "Perform valgrind memory testing upon 'make ctest' ?"
     FORCE)
 
-IF(NOT DEFINED LIBDIR_SUFFIX)
-    SET(LIBDIR_SUFFIX "")
-ENDIF()
-SET(LIBDIR_SUFFIX ${LIBDIR_SUFFIX}
-    CACHE STRING "Define suffix of lib directory name (empty string or 32 or 64)."
-    FORCE)
-
 IF(NOT DEFINED DEPENDENCY_SEARCH_PREFIX)
     SET(DEPENDENCY_SEARCH_PREFIX "")
 ENDIF()