configure: do not use character classes with awk

Some broken versions of awk (mawk, apparently used in debian)
do not handle character classes in regular expressions.

Originally committed as revision 21334 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -2223,7 +2223,7 @@ enabled pic && enable_pic
 check_cc <<EOF || die "Symbol mangling check failed."
 int ff_extern;
 EOF
-sym=$($nm -g $TMPO | awk '/ff_extern/{ print substr($0, match($0, /[^[:blank:]]*ff_extern/)) }')
+sym=$($nm -g $TMPO | awk '/ff_extern/{ print substr($0, match($0, /[^ \t]*ff_extern/)) }')
 extern_prefix=${sym%%ff_extern*}
 
 check_cc <<EOF && enable inline_asm