vpnc: fix musl compatibility

- Add missing `sys/ttydefaults.h` include to `config.c`
 - Restrict usage of `error.h` to glibc and uclibc only
 - Avoid including `linux/if_tun.h` on non-glibc/uclibc to prevent
   musl kernel/libc header conflicts

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

net/vpnc/patches/001-cross.patch

@@ -1,7 +1,7 @@
 --- a/Makefile
 +++ b/Makefile
 @@ -20,7 +20,7 @@
- # $Id$
+ # $Id: Makefile 539 2013-12-04 13:41:04Z Antonio Borneo $
  
  DESTDIR=
 -PREFIX=/usr/local
@@ -9,7 +9,7 @@
  ETCDIR=/etc/vpnc
  BINDIR=$(PREFIX)/bin
  SBINDIR=$(PREFIX)/sbin
-@@ -57,18 +57,15 @@
+@@ -57,18 +57,15 @@ OBJS = $(addsuffix .o,$(basename $(SRCS)
  CRYPTO_OBJS = $(addsuffix .o,$(basename $(CRYPTO_SRCS)))
  BINOBJS = $(addsuffix .o,$(BINS))
  BINSRCS = $(addsuffix .c,$(BINS))
@@ -31,7 +31,7 @@
  LIBS += -lnsl -lresolv -lsocket
  endif
  ifneq (,$(findstring Apple,$(shell $(CC) --version)))
-@@ -82,7 +79,7 @@
+@@ -82,7 +79,7 @@ vpnc : $(OBJS) vpnc.o
  	$(CC) $(LDFLAGS) -o $@ $^ $(LIBS)
  
  vpnc.8 : vpnc.8.template makeman.pl vpnc