cmake: check and show SYS_ALLOC_SIZE

CMakeLists.txt

@@ -18,6 +18,9 @@ endif(CMAKE_COMPILER_IS_GNUCC)
 
 if(WITH_SYSTEM_CALLS STREQUAL "mmap")
   set(PAGESIZE_ALIGN ON)
+  if(NOT DEFINED SYS_ALLOC_SIZE)
+    message(FATAL_ERROR "You have to set SYS_ALLOC_SIZE by using -DSYS_ALLOC_SIZE={int}.")
+  endif(NOT DEFINED SYS_ALLOC_SIZE)
 endif(WITH_SYSTEM_CALLS STREQUAL "mmap")
 
 # Checking free-list setups
@@ -115,6 +118,10 @@ endif (WITH_DOC)
 message(STATUS "********************************************")
 message(STATUS "********** ${PROJECT_NAME} build options : **********")
 message(STATUS "OS call for memory requests: " ${WITH_SYSTEM_CALLS})
+if(NOT WITH_SYSTEM_CALLS STREQUAL "none")
+  message(STATUS "Default allocation size for OS calls: " ${SYS_ALLOC_SIZE} " bytes")
+endif(NOT WITH_SYSTEM_CALLS STREQUAL "none")
+
 message(STATUS "POSIX locking mechanisms: " ${HAVE_LOCKS})
 
 message(STATUS "Raw blocks type: " ${RAW_BLOCKS_TYPE})