Move extra libtool options into configure

This should allow to keep the libtool options all in one place and at
the same time define different options depending on the host.

Made sure that -no-undefined is set only on Win32. Although no side
effects on Linux and OSX have been observed so far, it's probably better
to play it safe; it does not seem to be needed/does not seem to matter on *nix,
only required for Win32.

configure.ac

@@ -10,6 +10,17 @@ AM_INIT_AUTOMAKE([1.10 -Wall])
 m4_ifdef([AM_SILENT_RULES], [AM_SILENT_RULES([yes])])
 AC_CONFIG_MACRO_DIR([m4])
 
+LIBTOXCORE_LT_VERSION=0:0:0
+dnl
+dnl            current:revision:age
+dnl
+dnl current:  increment if interfaces have been added, removed or changed
+dnl revision: increment if source code has changed, set to zero if current is
+dnl           incremented
+dnl age:      increment if interfaces have been added, set to zero if
+dnl           interfaces have been removed or changed
+EXTRA_LT_LDFLAGS="-version-info $LIBTOXCORE_LT_VERSION"
+
 if test "x${prefix}" = "xNONE"; then
     prefix="${ac_default_prefix}"
 fi
@@ -106,6 +117,7 @@ case $host_os in
         WIN32="yes"
         AC_MSG_WARN([nTox is not supported on $host_os yet, disabling])
         BUILD_NTOX="no"
+        EXTRA_LT_LDFLAGS="$EXTRA_LT_LDFLAGS -no-undefined"
     ;;
     *solaris*)
         LIBS="$LIBS -lssp -lsocket -lnsl"
@@ -118,6 +130,8 @@ case $host_os in
 esac
 AM_CONDITIONAL(WIN32, test "x$WIN32" = "xyes")
 
+AC_SUBST(EXTRA_LT_LDFLAGS)
+
 # Checks for libraries.
 LIBSODIUM_LIBS=
 LIBSODIUM_LDFLAGS=

toxcore/Makefile.inc

@@ -1,5 +1,3 @@
-LIBTOXCORE_VERSION = 0:0:0
-
 lib_LTLIBRARIES = libtoxcore.la
 
 libtoxcore_la_include_HEADERS = \
@@ -33,8 +31,7 @@ libtoxcore_la_CFLAGS =  -I$(top_srcdir) \
                         -I$(top_srcdir)/toxcore \
                         $(LIBSODIUM_CFLAGS)
 
-libtoxcore_la_LDFLAGS = -version-info $(LIBTOXCORE_VERSION) \
-                        -no-undefined \
+libtoxcore_la_LDFLAGS = $(EXTRA_LT_LDFLAGS) \
                         $(LIBSODIUM_LDFLAGS) \
                         $(WINSOCK2_LIBS)