MDEV-25090: Depend on libpmem where available

WITH_PMEM: Change the parameter from BOOL to a ternary STRING
with a default value that allows to use libpmem when it is available.
In this way, a libpmem dependency will be automatically installed
on other than Debian-based systems, based on what is available on
the continuous integraton system for building the packages.

WITH_PMEM=auto (default): Use libpmem when available.
WITH_PMEM=yes: Require libpmem to be available.
WITH_PMEM=no: Do not depend on libpmem.

cmake/pmem.cmake

 INCLUDE(CheckIncludeFiles)
-OPTION(WITH_PMEM "Enable persistent memory features" OFF)
-IF(WITH_PMEM)
+SET(WITH_PMEM "auto" CACHE STRING "Enable persistent memory features")
+IF(WITH_PMEM STREQUAL "yes" OR WITH_PMEM STREQUAL "auto")
   FIND_LIBRARY(LIBPMEM pmem)
   CHECK_INCLUDE_FILES(libpmem.h HAVE_LIBPMEM_H)
   IF (NOT LIBPMEM)
-    MESSAGE(FATAL_ERROR "Can't find libpmem")
+    IF(WITH_PMEM STREQUAL "yes")
+      MESSAGE(FATAL_ERROR "Can't find libpmem")
+    ENDIF()
+    UNSET(HAVE_LIBPMEM_H CACHE)
+    UNSET(LIBPMEM CACHE)
   ELSEIF(NOT HAVE_LIBPMEM_H)
-    MESSAGE(FATAL_ERROR "Can't find libpmem.h")
+    IF(WITH_PMEM STREQUAL "yes")
+      MESSAGE(FATAL_ERROR "Can't find libpmem.h")
+    ENDIF()
+    UNSET(HAVE_LIBPMEM_H CACHE)
+    UNSET(LIBPMEM CACHE)
   ELSE()
     ADD_DEFINITIONS(-DHAVE_PMEM)
   ENDIF()
+ELSEIF(WITH_PMEM STREQUAL "no")
+  UNSET(HAVE_LIBPMEM_H CACHE)
+  UNSET(LIBPMEM CACHE)
+ELSE()
+  MESSAGE(FATAL_ERROR "Invalid value for WITH_PMEM. Must be 'yes', 'no', or 'auto'.")
 ENDIF()