[cpan #49618] fix odd win32 nmake but gcc case

Apperently according to cpan #49618 somebody tried to use activeperl with gcc.
So check for cl also.
fixed decompression of empty strings, broken since 94e6dedbcf

94e6dedbcf support memBunzip BZh header w/o extra size prefix
had the special case with out_len = 0. But out_len = 0 is also used for empty
strings. Use a new noprefix var instead.