[httpd] Fix building on FreeBSD which doesn't have SYS_gettid

configure.ac

@@ -54,6 +54,12 @@ AC_CHECK_FUNCS_ONCE([posix_fadvise pipe2 syscall])
 AC_CHECK_FUNCS([strptime strtok_r], [],
 	[AC_MSG_ERROR([[Missing function required to build OwnTone]])])
 
+dnl FreeBSD doesn't have SYS_gettid
+AC_CHECK_DECL([SYS_gettid],
+	[AC_DEFINE([HAVE_SYS_GETTID], 1,
+		[Define to 1 if SYS_gettid exists])],
+	[], [#include <sys/syscall.h>])
+
 dnl check for clock_gettime or replace it
 AC_SEARCH_LIBS([clock_gettime], [rt],
 	[AC_DEFINE([HAVE_CLOCK_GETTIME], 1,

src/httpd.c

@@ -34,7 +34,7 @@
 #include <stdint.h>
 #include <inttypes.h>
 
-#ifdef HAVE_SYSCALL
+#ifdef HAVE_SYS_GETTID
 #include <sys/syscall.h> // get thread ID
 #endif
 
@@ -853,7 +853,7 @@ request_async_cb(void *arg)
 {
   struct httpd_request *hreq = *(struct httpd_request **)arg;
 
-#ifdef HAVE_SYSCALL
+#if defined(HAVE_SYSCALL) && defined(HAVE_SYS_GETTID)
   DPRINTF(E_DBG, hreq->module->logdomain, "%s request '%s' in worker thread %ld\n", hreq->module->name, hreq->uri, syscall(SYS_gettid));
 #endif