Allow cxxflags to be defined as empty

[madmongo1]

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Environment and version details

All

Describe your use case

When writing CI scripts it is useful to be able to specify some compiler flags in a matrix variable.
If for a given row in the matrix, there are no flags to set this becomes a problem because currently cxxflags=
is an invalid command line component for b2.

A workaround is to include the cxxflags in the matrix variable, i.e.

cxxflags: "cxxflags=-coverage"
...
cxxflags: ""

followed by:

run: b2 <options...> ${{matrix.cxxflags}} <more options...> <targets>

But this feels suboptimal as it splits logic across the script.

Describe the solution you'd like

I would like this to work:

b2 cxxflags= target

Which would then build target with no additional command line flags set. i.e. it would be identical to writing:

b2 target

[grafikrobot]

Note for future reference: This would likely be implemented in the property argument parsing code. It would need to translate feature-name= to either: ignoring the argument for optional features as the value is empty, or allowing an empty value for free features instead an error. Doing the second one is problematic as it would be a likely breaking change as it contrary to existing practice. Which, of note, would be the case for this request as cxxflags is a free feature.