Add INSTALL_ROOT support for mythffmpeg/play

Backported from master.   I had to filter out the BINDIR, etc from the mythtv
config.mak which we promptly override anyways.

mythtv/configure

@@ -4772,7 +4772,8 @@ SLIB_EXTRA_CMD=${SLIB_EXTRA_CMD}
 SLIB_INSTALL_EXTRA_CMD=${SLIB_INSTALL_EXTRA_CMD}
 SLIB_UNINSTALL_EXTRA_CMD=${SLIB_UNINSTALL_EXTRA_CMD}
 EOF
-cat >> external/FFmpeg/config.mak < $MYTH_CONFIG_MAK
+# Strip out the old dirs that we are about to put back in.
+sed -e '/^S?H?LIBDIR=.*$/d' -e '/^INCDIR=.*$/d' -e '/^BINDIR=.*$/d' -e '/^DATADIR=.*$/d' -e '/^MANDIR=.*$/d' $MYTH_CONFIG_MAK >> external/FFmpeg/config.mak
 cat >> external/FFmpeg/config.mak <<EOF
 LIBDIR=\$(INSTALL_ROOT)$libdir
 SHLIBDIR=\$(INSTALL_ROOT)$shlibdir