autoscan is getting a warning from perl on this RE. Fix it.

Should pass this upstream.

--- autoconf-2.69/bin/autoscan.in_orig 2016-03-15 17:17:51.090635380 -0700

+++ autoconf-2.69/bin/autoscan.in 2016-03-15 17:18:10.667432500 -0700

@@ -358,7 +358,7 @@

{

# Strip out comments and variable references.

s/#.*//;

- s/\${[^\}]*}//g;

+ s/\$\{[^\}]*}//g;

s/@[^@]*@//g;

# Tokens in the code.