issue: #21718

build break reported:
crypto/threads_pthread.c:76:5: warning: implicit declaration of function 'pthread_mutexattr_settype'; did you mean 'pthread_mutexattr_destroy'? [-Wimplicit-function-declaration]
   76 |     pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_NORMAL);
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~
      |     pthread_mutexattr_destroy
crypto/threads_pthread.c:76:38: error: 'PTHREAD_MUTEX_NORMAL' undeclared (first use in this function); did you mean 'PTHREAD_MUTEX_TIMED_NP'?
   76 |     pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_NORMAL);
      |                                      ^~~~~~~~~~~~~~~~~~~~
      |                                      PTHREAD_MUTEX_TIMED_NP

This occurs because PTHREAD_MUTEX_NORMAL is only defined in glibc if
__USE_UNIX98 or __USE_XOPEN2K8 is defined, which is derived from setting
__USE_POSIX_C_SOURCE or __XOPEN_SOURCE is selected in the glibc feature
set for a build.  Since openssl selects no specific feature set from
glibc, the build break occurs

We could select a feature set of course, but that seems like a
significant discussion to have prior to doing so. Instead, the simpler
solution is to just not set the mutex type at all, given that
pthread_mutexattr_init sets the default mutex type, which should be akin
to normal anyway (i.e. no mutex error checking or allowed-recursive
behavior)

Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from #21726)

(cherry picked from commit e4d8086)