configure: Set the default for --shlibdir to --libdir.

Fixes ticket #4183.

Reviewed-by: Ivan Kalvachev

configure

@@ -84,7 +84,7 @@ Standard options:
   --datadir=DIR            install data files in DIR [PREFIX/share/ffmpeg]
   --docdir=DIR             install documentation in DIR [PREFIX/share/doc/ffmpeg]
   --libdir=DIR             install libs in DIR [PREFIX/lib]
-  --shlibdir=DIR           install shared libs in DIR [PREFIX/lib]
+  --shlibdir=DIR           install shared libs in DIR [LIBDIR]
   --incdir=DIR             install includes in DIR [PREFIX/include]
   --mandir=DIR             install man page in DIR [PREFIX/share/man]
   --enable-rpath           use rpath to allow installing libraries in paths
@@ -2685,7 +2685,7 @@ docdir_default='${prefix}/share/doc/ffmpeg'
 incdir_default='${prefix}/include'
 libdir_default='${prefix}/lib'
 mandir_default='${prefix}/share/man'
-shlibdir_default="$libdir_default"
+shlibdir_default='${LIBDIR}'
 
 # toolchain
 ar_default="ar"