Inverting the logic to set the prefix for SYSCONF_INSTALL_DIR

cmake/modules/DefineInstallationPaths.cmake

@@ -73,7 +73,7 @@ if (UNIX)
     CACHE PATH "The XDG directory"
   )
 
-  IF("${EXEC_INSTALL_PREFIX}" STREQUAL "/usr")
+  IF(NOT "${EXEC_INSTALL_PREFIX}" STREQUAL "/usr")
     SET(SYSCONFDIR_INSTALL_PREFIX "${EXEC_INSTALL_PREFIX}")
   endif()