This is a fix specific for HPUX,
for which the compiler does not resolve properly dependencies
involving unused inline functions.
(See existing comments in mysql_thread.h)

In include/mysql/psi/mysql_thread.h,
the instrumentation helpers for mysql_prlock_*
uses the pr lock apis.

These apis are implemented in mysys/thr_rwlock.c,
which is not linked to client code.

As a result, the code does not link in libmysql_r, on HPUX.

The fix is to cut dependencies explicitely,
by introducing -DDISABLE_MYSQL_RWLOCK_H,
when building client code.