Added support for packet compression, thanks to Mark Glines.

Add "Compression = <level>" to the host config files, where level can be
0 (off), or any integer between 1 (fast) and 9 (best).

m4/zlib.m4

@@ -0,0 +1,31 @@
+dnl Check to find the zlib headers/libraries
+
+AC_DEFUN(tinc_ZLIB,
+[
+  tinc_ac_save_CPPFLAGS="$CPPFLAGS"
+
+  AC_ARG_WITH(zlib-include,
+    [  --with-zlib-include=DIR  zlib headers directory],
+    [zlib_include="$withval"
+     CFLAGS="$CFLAGS -I$withval"
+     CPPFLAGS="$CPPFLAGS -I$withval"]
+  )
+
+  AC_ARG_WITH(zlib-lib,
+    [  --with-zlib-lib=DIR  zlib library directory],
+    [zlib_lib="$withval"
+     LIBS="$LIBS -L$withval"]
+  )
+
+  AC_CHECK_HEADERS(zlib.h,
+    [],
+    [AC_MSG_ERROR("zlib header files not found."); break]
+  )
+
+  CPPFLAGS="$tinc_ac_save_CPPFLAGS"
+
+  AC_CHECK_LIB(z, compress2,
+    [LIBS="$LIBS -lz"],
+    [AC_MSG_ERROR("zlib libraries not found.")]
+  )
+])