Revert the merger of LC_ALL declaration and export.

This part of the configure script might be run by non-POSIX shells.

Originally committed as revision 15599 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -8,7 +8,8 @@
 #
 
 # Prevent locale nonsense from breaking basic text processing.
-export LC_ALL=C
+LC_ALL=C
+export LC_ALL
 
 # make sure we are running under a compatible shell
 # try to make this part work with most shells