configure: fix print_config() with broke awks

Some awk versions do not treat the result of unary + on a (numeric)
string as numeric, giving wrong results when used in a boolean context
Using unary - instead is logically equivalent works as expected.

Signed-off-by: Mans Rullgard <mans@mansr.com>

configure

@@ -630,7 +630,7 @@ print_config(){
                 } else if (file ~ /\\.asm\$/) {
                     printf(\"%%define %s %d\\n\", c, v) >>file;
                 } else if (file ~ /\\.mak\$/) {
-                    n = +v ? \"\" : \"!\";
+                    n = -v ? \"\" : \"!\";
                     printf(\"%s%s=yes\\n\", n, c) >>file;
                 }
             }