Added check for pwd.h.

diff --git a/configure.ac b/configure.ac
index 8f20a46..35abff4 100644
--- a/configure.ac
+++ b/configure.ac
@@ -907,6 +907,7 @@
 	netinet/ip.h \
 	netinet/tcp.h \
 	pty.h \
+	pwd.h \
 	resolv.h \
 	sched.h \
 	scsi/sg.h \