Improved the capabilities feature detection. Previously included file sys/capability.h mentioned in capset(2) man page, belongs to the libcap-dev package, which may not be installed on some Linux systems when compiling nginx. This prevented the capabilities feature from being detected and compiled on that systems. Now linux/capability.h system header is included instead. Since capset() declaration is located in sys/capability.h, now capset() syscall is defined explicitly in code using the SYS_capset constant, similarly to other Linux-specific features in nginx.

--- a/src/os/unix/ngx_process_cycle.c
+++ b/src/os/unix/ngx_process_cycle.c
@@ -869,7 +869,7 @@ ngx_worker_process_init(ngx_cycle_t *cyc
             data.effective = CAP_TO_MASK(CAP_NET_RAW);
             data.permitted = data.effective;
 
-            if (capset(&header, &data) == -1) {
+            if (syscall(SYS_capset, &header, &data) == -1) {
                 ngx_log_error(NGX_LOG_EMERG, cycle->log, ngx_errno,
                               "capset() failed");
                 /* fatal */