Add support for flags with a composite value.

[jurko.gospodnetic@…]

Add support for flags constructing their value from a some fixed string part as well as one or more feature values.

Currently it is possible to define a flag that uses a fixed predefined string as its value:

toolset.flags compile.action MY_FLAG <some-feature> : --my-fixed-compiler-option ;

Or it is possible to define a flag that uses a feature's value as its own using either of the following:

toolset.flags compile.action MY_FLAG <some-feature> ;
toolset.flags compile.action MY_FLAG <condition>xxx : <feature-whose-value-we-want> ;

But it is not possible to define a flag that would take a property like <template-depth>128 and construct '-ftemplate-depth-128' as its value.

As a workaround one can make the flag take only the property's value as its own and then add whatever text to it is needed to construct a proper command-line option in the action using the tool's value.

As I see it this support would not bring all that much to Boost Build except possibly some interface completeness. The only thing I can think off is that one could prepare a flag suitable for multiple actions and then not duplicate the code for constructing the actual command-line option in each of those actions. For an example see gcc.compile.c++ & gcc.compile.c++.pch rules in Boost Build's tools/gcc.jam toolset module.

I believe the key to implementing this feature is defining the syntax used to define such flags and then modifying the toolset.handle-flag-value rule accordingly.

If & when this support is added it can be used to merge existing flags like TEMPLATE_DEPTH into more generic ones like OPTIONS where they do seem to fit in nicely and have not already been merged with them only due to this missing support.