Documentation Error when building from source

[DLu]

Environment

• OS Version: 20.04
• Source or binary build? Source, ign-gui5 branch

Description

• Expected behavior: When executing the command colcon build --merge-install --cmake-args -DBUILD_TESTING=OFF I expected the build to complete.
• Actual behavior: Failure to finish building ignition-gui5 with the following output:

--- stderr: ignition-gui5                                 
error: Illegal format for option TAGFILES, no name specified for item '='
error: Illegal format for option TAGFILES, no name specified for item '='
error: Illegal format for option TAGFILES, no name specified for item '='
error: Illegal format for option TAGFILES, no name specified for item '='
error: Illegal format for option TAGFILES, no name specified for item '='
error: Illegal format for option TAGFILES, no name specified for item '='
error: Illegal format for option TAGFILES, no name specified for item '='
error: Illegal format for option TAGFILES, no name specified for item '='
---

[DLu]

Note, this was also referenced in gazebosim/gz-cmake#122 (comment)