Define a new CPP symbol that the target OS is Linux, and use it where only the OS matters and not the threads Library.
Until now, 'HAVE_LINUXTHREADS' was used to indicate "Target is Linux" in many places.
When we support configuration with NPTL but no Linuxthreads, this misuse must cease.


configure.in:
  Step 2 of the switch to support configuration with NPTL:
  Define a new CPP symbol that the target OS is Linux.
  Until now, 'HAVE_LINUXTHREADS' was used to indicate "Target is Linux" in many places.
  When we support configuration with NPTL but no Linuxthreads, this misuse must cease.
include/my_global.h:
  Step 2 of the switch to support configuration with NPTL:
  Use the new 'TARGET_OS_LINUX' where only the OS matters and not the threads Library.
mysys/thr_mutex.c:
  Step 2 of the switch to support configuration with NPTL:
  Use the new 'TARGET_OS_LINUX' where only the OS matters and not the threads Library.
sql/stacktrace.c:
  Step 2 of the switch to support configuration with NPTL:
  Use the new 'TARGET_OS_LINUX' where only the OS matters and not the threads Library.
sql/stacktrace.h:
  Step 2 of the switch to support configuration with NPTL:
  Use the new 'TARGET_OS_LINUX' where only the OS matters and not the threads Library.
tools/mysqlmanager.c:
  Step 2 of the switch to support configuration with NPTL:
  Use the new 'TARGET_OS_LINUX' where only the OS matters and not the threads Library.