Dynamic measurement thread issue

[jpcima]

The InitTables method of Dosbox to initialize global data has a defect when using the emulator on multiple threads.

Description of the problem:

• have 2 threads doing Init() each around the same time
• T1 has entered InitTables(), has set doneTables = true
• T2 takes over, InitTables() skips because of doneTables flag
• T2 generates bad audio because tables are not filled

Also, there must be a synchronization to ensure correct sequencing of read and write.
Mutex will be fine.

(EDIT I don't use several threads in the worker for measuring so it's not affected; but needed for libADLMIDI's genadldata and I'll copy the changes here)