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.

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@21334 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

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