ENH: Allow fine grained control over dependancies

ENH: Allow fine grained control over dependancies

An end user may want to explicitly avoid using
a feature that can be automatically found on the
build system.  This arose when the build machine
had libraries for LZMA but the target machine
did not have those libraries available.

By allowing flags the optional features provided by
LZMA/ZLIB/BZip2/EXPAT/PCREPOSIX/LibGCC
can be explicitly removed (Default behavior is
to use what can be found).