MINOR: cfgcond: support negating conditional expressions

Now preceeding a config condition term with "!" will simply negate it.
Example:

   .if !feature(OPENSSL)
       .alert "SSL support is mandatory"
   .endif
diff --git a/doc/configuration.txt b/doc/configuration.txt
index b128718..ccb77a5 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -811,6 +811,8 @@
   - the integer zero ('0'), always returns "false"
   - a non-nul integer (e.g. '1'), always returns "true".
   - a predicate optionally followed by argument(s) in parenthesis.
+  - a question mark ('!') preceeding any of the non-empty elements above, and
+    which will negate its status.
 
 The list of currently supported predicates is the following:
 
@@ -856,6 +858,10 @@
        profiling.memory on
    .endif
 
+   .if !feature(OPENSSL)
+       .alert "SSL support is mandatory"
+   .endif
+
 Four other directives are provided to report some status:
 
   - .diag "message"    : emit this message only when in diagnostic mode (-dD)