ITS#9215 Fix feature test macro for pthread_setconcurrency
Follow the pattern used elsewhere.
Tested on Debian 10 (buster), Solaris 10, and Solaris 11 (Solaris Studio 12.6 on both of those).
Fixes the following compile failure on Solaris 10:
libtool: compile: cc -g -I../../include -I../../include -DLDAP_R_COMPILE -I./../libldap -I/opt/csw/include -DLDAP_LIBRARY -c thr_posix.c -KPIC -DPIC -o .libs/thr_posix.o
"/usr/include/sys/feature_tests.h", line 337: #error: "Compiler or options invalid for pre-UNIX 03 X/Open applications and pre-2001 POSIX applications"
cc: acomp failed for thr_posix.c
*** Error code 1