mbrtowc: port to AIX 7.1 with xlc 12.1
Fix a problem with locks when building GNU Tar (Savannah commit
55f2a0772e08b9febac3ac0de5cb048d4c60d2f5) on AIX 7.1 with IBM XL
C/C++ V12.1 using ‘./configure CC=xlc’. The link fails due to
missing definitions of pthread_mutex_lock and
pthread_mutex_unlock. GNU Tar uses unlocked-io and so
should not need to worry about multithreading or locks.
* lib/mbtowc-lock.h (mbtowc_with_lock) [USE_UNLOCKED_IO]:
Don’t bother with locks, since this app is single-threaded.
There may be similar linking problems with lib/nl_langinfo.c and
lib/setlocale_null.c but my GNU Tar build didn’t run into them, so
I left them alone for now.