Add compiler warnings specification levels.

[dpkristensen]

Allow a package to describe a generic level of robustness to warnings in "classes" of compilers.

Compiler classes may include but are not limited to:

• arm
• msvc
• gcc

Warning levels per class are:

• None (default)
• Minimal
• Full

Whether the build system uses this information, or how is up to the implementation of the build system.

Exceptions can be allowed as well for specific packages or groups of packages:

• Add checking for a specific warning (Basic)
• Remove checking for a specific warning (Full)

Should also let the system configure the tool chain settings itself instead of using default options. Maybe default settings is something that has to be loaded?