Change VHOOKSHFLAGS (which defaults to SHFLAGS) from a shell to a Makefile

variable so that overriding SHFLAGS later in the configure script has the
desired effect.

Originally committed as revision 6281 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -448,7 +448,7 @@ dlfcn="no"
 dlopen="no"
 mpegaudio_hp="yes"
 SHFLAGS='-shared -Wl,-soname,$@'
-VHOOKSHFLAGS="$SHFLAGS"
+VHOOKSHFLAGS='$(SHFLAGS)'
 netserver="no"
 need_inet_aton="no"
 protocols="yes"