PROTON-2346: c/src/proactor/epoll-internal.h: fix build on musl (#300)
PROTON-2346: c/src/proactor/epoll-internal.h: fix build on musl (#300)

* PROTON-2346: c/src/proactor/epoll-internal.h: fix build on musl

Build with epoll proactor on musl is broken since at least version 0.23.0 and
https://github.com/apache/qpid-proton/commit/37136940e3077f25ce58c94775f48c66f666f4a8
because musl does not define PTHREAD_MUTEX_ADAPTIVE_NP resulting in the
following build failure:

In file included from /home/giuliobenetti/autobuild/run/instance-0/output-1/build/qpid-proton-0.33.0/c/src/proactor/epoll.c:60:
/home/giuliobenetti/autobuild/run/instance-0/output-1/build/qpid-proton-0.33.0/c/src/proactor/epoll-internal.h: In function 'pmutex_init':
/home/giuliobenetti/autobuild/run/instance-0/output-1/build/qpid-proton-0.33.0/c/src/proactor/epoll-internal.h:319:36: error: 'PTHREAD_MUTEX_ADAPTIVE_NP' undeclared (first use in this function); did you mean 'PTHREAD_MUTEX_STALLED'?
  319 |   pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_ADAPTIVE_NP);
      |                                    ^~~~~~~~~~~~~~~~~~~~~~~~~
      |                                    PTHREAD_MUTEX_STALLED

Fixes:
 - http://autobuild.buildroot.org/results/6a901b9ff68b7f52cabf8273d1017025fbd93b0d

Signed-off-by: Fabrice Fontaine <7d88633730b8b85c1516e612893df72eae9cd341@gmail.com>

* Update c/src/proactor/epoll-internal.h with explanatory comment

Co-authored-by: Fabrice Fontaine <7d88633730b8b85c1516e612893df72eae9cd341@gmail.com>
apache/qpid-proton,ChugR/qpid-proton,ssorj/qpid-proton,astitcher/qpid-proton,gemmellr/qpid-proton,ChugR/qpid-proton,apache/qpid-proton,ChugR/qpid-proton,ssorj/qpid-proton,ChugR/qpid-proton,apache/qpid-proton,ssorj/qpid-proton,apache/qpid-proton,gemmellr/qpid-proton,gemmellr/qpid-proton,astitcher/qpid-proton,ChugR/qpid-proton,ssorj/qpid-proton,ssorj/qpid-proton,ChugR/qpid-proton,apache/qpid-proton,gemmellr/qpid-proton,gemmellr/qpid-proton,ssorj/qpid-proton,astitcher/qpid-proton,apache/qpid-proton,astitcher/qpid-proton,gemmellr/qpid-proton,astitcher/qpid-proton,astitcher/qpid-proton