Generate a DT_SONAME on *BSD machines

http://bugs.gentoo.org/123571

--- configure
+++ configure
@@ -80,7 +80,7 @@
   SFLAGS="${CFLAGS-"-O3"} -fPIC"
   CFLAGS="$cflags"
   case `(uname -s || echo unknown) 2>/dev/null` in
-  Linux | linux | GNU | GNU/*)
+  Linux | linux | GNU | GNU/* | *BSD | DragonFly)
           LDSHARED=${LDSHARED-"$cc -shared -Wl,-soname,libz.so.1"}
           MAPFILE=${MAPFILE-"-Wl,--version-script,mapfile"};;
   CYGWIN* | Cygwin* | cygwin* | OS/2* )