allow nc to build on linux and os x

apps/nc/Makefile.am

@@ -1,8 +1,6 @@
 include $(top_srcdir)/Makefile.am.common
 
-if HOST_OPENBSD
-
-if HAVE_POLL
+if BUILD_NC
 
 noinst_PROGRAMS = nc
 
@@ -11,15 +9,26 @@ EXTRA_DIST = nc.1
 nc_LDADD = $(PLATFORM_LDADD) $(PROG_LDADD)
 nc_LDADD += $(top_builddir)/tls/libtls.la
 
+CPPFLAGS += -I$(top_srcdir)/apps/nc/compat
+
 nc_SOURCES = atomicio.c
 nc_SOURCES += netcat.c
 nc_SOURCES += socks.c
 noinst_HEADERS = atomicio.h
+noinst_HEADERS += compat/sys/socket.h
+
+nc_SOURCES += compat/socket.c
+
+if !HAVE_ACCEPT4
+nc_SOURCES += compat/accept4.c
+endif
+
+if !HAVE_READPASSPHRASE
+nc_SOURCES += compat/readpassphrase.c
+endif
 
 if !HAVE_STRTONUM
-nc_SOURCES += strtonum.c
-endif
-
+nc_SOURCES += compat/strtonum.c
 endif
 
 endif