Do not rely on an implicit function declaration for detecting
the daemon function.  Future compilers may not accept such
declarations by default, causing the detection result to change.

Submitted upstream: <https://github.com/NLnetLabs/dnssec-trigger/pull/11>

diff --git a/configure b/configure
index 079ea641e2940515..22c9487fb0d311f8 100755
--- a/configure
+++ b/configure
@@ -6757,6 +6757,7 @@ else
 
 echo '
 #include <stdlib.h>
+#include <unistd.h>
 ' >conftest.c
 echo 'void f(){ (void)daemon(0, 0); }' >>conftest.c
 if test -z "`$CC -c conftest.c 2>&1 | grep deprecated`"; then
diff --git a/configure.ac b/configure.ac
index c809367d307f108e..e8095fe7288ba68a 100644
--- a/configure.ac
+++ b/configure.ac
@@ -225,6 +225,7 @@ AC_CHECK_FUNCS([daemon])
 if test $ac_cv_func_daemon = yes; then
         ACX_FUNC_DEPRECATED([daemon], [(void)daemon(0, 0);], [
 #include <stdlib.h>
+#include <unistd.h>
 ])
 fi