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