Update: Get clang working by separating out gcc specific settings, such as -fstrict...

Update: Get clang working by separating out gcc specific settings, such as -fstrict-flex-arrays=3, and related changes.

The "-fstrict-flex-arrays=3" flag is not supported by clang.
Create a new mode call "gcc", make that the default, and move the "-fstrict-flex-arrays=3" into the gcc-specific setting.

Update the unit tests and mocks with these changes.
Make the unit tests and mocks more consistent by having these same settings.

Add a missing period at the end of the clang specific mode documentation comment.

The example bootstrap script handling of threads is improved.