Zabbix was configured with --enable-agent. Running make yields:

Making all in src
Making all in libs
...
Making all in zabbix_agent
gcc -DHAVE_CONFIG_H -I. -I../../include -g -O2 -MT vmstats.o -MD -MP -MF .deps/vmstats.Tpo -c -o vmstats.o vmstats.c
vmstats.c: In function `update_vmstat':
vmstats.c:213: error: `_system_configuration' undeclared (first use in this function)
vmstats.c:213: error: (Each undeclared identifier is reported only once

One hypothesis is that gcc is the culprit, because it did compile earlier with cc.