Support some form of conditional compilation

[hopscc]

Allow Code to be conditionally compiled in cobra on at least:
Debugging on/off
Backend compiler in use
OS platform
Whether tests, assertions and contracts enabled or not.

Perhaps also from a user defined setting passed to the compiler at
compilation.

Simplify greatly platform specific code ( backend or OS) layout.

See also  extending help directive forum topic

[hopscc]

tag-match-block conditional compile code -
support platform, backend, compiler settings
debug and triggering off env vars.
Tests for same...
Proof of concept code only.

Needs added code for 'otherwise' cases,
cobra version-no matching/exposure and
ability to pass defined variable/value from compiler commandline.

[hopscc]

Extend and replacement for first patch file:

'otherwise' tags using 'otherThan'
matching version numbers for cobra and clr
user specified tags and -versionTag/define commandline switch

Some tidy up of version code in commandLine.
More tests.

I'm done with this for the moment so I'll reassign ownership.