guard provisional extensions with an opt-in define

[bashbaug]

fixes #247

Wraps provisional extensions in an opt-in define so applications that do not use provisional extensions do not get broken by non-backwards compatible changes.

Notes:

• The define gets added automatically during header generation based on the provisional="true" line in the XML file.
• The define is currently an opt-in named CL_PROVISIONAL_EXTENSIONS, but it's trivial to change this if desired.
• Because it is an opt-in define, the current implementation does break backwards compatiblity, and applications using provisional extensions will need to add the opt-in define.
• We should consider how much additional testing we want to add (in this repo and in others) with this change. For example, do we want to run some of the header tests with and without provisional extensions?