Buildsystem/CMake: Add the ability to avoid linking the core with jemalloc on linux through using the -DNOJEM parameter.

WARNING: This is a developer flag only, and as such NOT documented elsewhere - we need this for valgrind and related tools.
Again : YOU HAVE BEEN WARNED! DO NOT USE THIS FLAG UNLESS YOU REALLY KNOW WHAT YOU ARE DOING!

cmake/showoptions.cmake

@@ -78,4 +78,10 @@ if( WIN32 )
   endif()
 endif( WIN32 )
 
-message("")
+if ( NOJEM )
+  message("")
+  message("*** WARNING: jemalloc linking has been disabled!")
+  message("*** Please note that this is for DEBUGGING WITH VALGRIND only!")
+  message("*** DO NOT DISABLE IT UNLESS YOU KNOW WHAT YOU'RE DOING!")
+endif()
+  message("")