Fix mixin guard comparison operator list

The operator list includes "=<", but not "<>", "!=" or "<=". Properly
adjust test cases and drop occurences of invalid operators. Raise a
syntax error if an unknown operator is used. See
http://lesscss.org/#-pattern-matching-and-guard-expressions
Deferred variables inside @media identifiers

Followup of 11895c
Fix block fills generation with only variables.

If the parsed block contents contains only LESS variable definitions,
the block should not be renderend. Previously this was implemented by
not returning self from Variable.parse() (contrary to it's docstring and
other plib classes). The real fix is to just check for the above
condition and return self again.
Not recurse forever if missing mixin declarations

Bail out after recursing 10 times into Deferred.parse and raise syntax
error for missing declaration to provide a meaningful error message.
Otherwise the code loops until the interpreter stack end (max
recursion depth).