unbreak compilation with vhook enabled

the backported introduced a regression that prevents successfully
linking vhook shared objects on systems with gnu linkers. As the version
scripts only apply to shared objects against that applications are being
linked, this commit unties the VHOOKSHFLAGS and SHFLAGS variable.


Originally committed as revision 21609 to svn://svn.ffmpeg.org/ffmpeg/branches/0.5

configure

@@ -1201,7 +1201,7 @@ vhook="default"
 # build settings
 add_cflags -D_ISOC99_SOURCE -D_POSIX_C_SOURCE=200112
 SHFLAGS='-shared -Wl,-soname,$$(@F)'
-VHOOKSHFLAGS='$(SHFLAGS)'
+VHOOKSHFLAGS='-shared -Wl,-soname,$$(@F)'
 FFSERVERLDFLAGS=-Wl,-E
 LIBPREF="lib"
 LIBSUF=".a"