perl plugin: fix build on RHEL5

perl plugin: fix build on RHEL5

perl.c: In function 'init_pi':
perl.c:2304: warning: implicit declaration of function
'pthread_mutexattr_settype'
perl.c:2304: error: 'PTHREAD_MUTEX_RECURSIVE' undeclared (first use in
this function)
perl.c:2304: error: (Each undeclared identifier is reported only once
perl.c:2304: error: for each function it appears in.)
make[3]: *** [perl_la-perl.lo] Error 1