Include <unistd.h> for the setresuid and setresgid functions,
to avoid an implicit function declaration.

Submitted upstream: <https://github.com/opendnssec/opendnssec/pull/843>

diff --git a/configure b/configure
index bf515cde3d4fab71..52d2885d6a6ef546 100755
--- a/configure
+++ b/configure
@@ -21101,6 +21101,7 @@ else
   cat confdefs.h - <<_ACEOF >conftest.$ac_ext
 /* end confdefs.h.  */
 
+#include <unistd.h>
 #include <stdlib.h>
 #include <errno.h>
 int main(){errno=0; setresuid(0,0,0); if (errno==ENOSYS) exit(1); else exit(0);}
@@ -21143,6 +21144,7 @@ else
   cat confdefs.h - <<_ACEOF >conftest.$ac_ext
 /* end confdefs.h.  */
 
+#include <unistd.h>
 #include <stdlib.h>
 #include <errno.h>
 int main(){errno=0; setresgid(0,0,0); if (errno==ENOSYS) exit(1); else exit(0);}
diff --git a/m4/acx_broken_setres.m4 b/m4/acx_broken_setres.m4
index 374cee0b0b8ef196..467db9170a319170 100644
--- a/m4/acx_broken_setres.m4
+++ b/m4/acx_broken_setres.m4
@@ -4,6 +4,7 @@ AC_DEFUN([ACX_BROKEN_SETRES],[
 		AC_MSG_CHECKING(if setresuid seems to work)
 		AC_RUN_IFELSE(
 			[AC_LANG_SOURCE([[
+#include <unistd.h>
 #include <stdlib.h>
 #include <errno.h>
 int main(){errno=0; setresuid(0,0,0); if (errno==ENOSYS) exit(1); else exit(0);}
@@ -20,6 +21,7 @@ int main(){errno=0; setresuid(0,0,0); if (errno==ENOSYS) exit(1); else exit(0);}
 		AC_MSG_CHECKING(if setresgid seems to work)
 		AC_RUN_IFELSE(
 			[AC_LANG_SOURCE([[
+#include <unistd.h>
 #include <stdlib.h>
 #include <errno.h>
 int main(){errno=0; setresgid(0,0,0); if (errno==ENOSYS) exit(1); else exit(0);}