Fix expanding undefined el6 macro
    
    The spec file could not been processed on Fedora:
    
    $ fedpkg local
    error: parse error in expression
    error: /home/test/fedora/perl-Encode-EUCJPASCII/perl-Encode-EUCJPASCII.spec:19: bad %if condition
    error: query of specfile /home/test/fedora/perl-Encode-EUCJPASCII/perl-Encode-EUCJPASCII.spec failed, can't parse
    
    This fixes it by expanding the macro only if it is defined.