Compilation error if pthread_mutexattr_set_type() is not present

[bennylp]

PJLIB's os_core_unix.c needs to set the mutex type to either simple or recursive, and this is done by calling pthread_mutexattr_set_type().

Unfortunately pthread_mutexattr_set_type() may not be available in all cases (e.g. in Newlib, IIRC), and this will cause compilation to fail.

The configure script should detect if this feature is available and set the appropriate flags.

[bennylp]

Fixed in r1783:

• Added PJ_HAS_PTHREAD_MUTEXATTR_SETTYPE macro which is checked by the configure script. This macro specifies whether pthread_mutexattr_settype() is present.
• Also added PJ_PTHREAD_MUTEXATTR_T_HAS_RECURSIVE macro which is checked by the configure script. This macro specifies whether pthread_mutexattr_t as the recursive field.
• If pthread_mutexattr_settype() is not present, and when recursive mutex is needed, os_core_unix.c will set pthread_mutexattr_t.recursive to 1 if PJ_PTHREAD_MUTEXATTR_T_HAS_RECURSIVE macro is set.