Support for position-independent code generation (linking static boost libraries into shared libraries with -fPIC)

[jeking3]

From a discussion in Boost.Serialization and on the mailing list, it was understood that the gcc compiler jam enables -fPIC if linking shared. Another use case is linking static, but then incorporating that into a shared library. On some platform/compiler combinations this requires special treatment, for example with linux and gcc or clang, -fPIC is required to enable this to work.

From the mailing list I received advice on testing for -fPIC support in the compiler and then enabling it on a given project. @pdimov suggested that to do it properly would require some work in Boost.Build, but I think it's worth the effort.

Steven Watanabe wrote:
...

import flags ;
project
: requirements
[ check-has-flag -fPIC : -fPIC ]
;

Note 1: The documentation for this is only in
b2 --help flags.check-has-flag. It isn't
integrated into the html docs yet.

Note 2: This is pretty new, so if you run into
any problems, please let me know.

Note 3: It might be more reliable to separate
the object files out and set shared on them:
obj x.o : x.cpp : shared ;
...
lib l : x.o y.o ... ;
(This will only work if the library is one
that is specifically built for the test, not
if it's a generic Boost library.)

Note 4: (from @pdimov)

This should probably be a feature, like <position-independent>on, which
toolsets translate to -fPIC as appropriate.

[stale]

Thank you for your contributions. Main development of B2 has moved to https://github.com/bfgroup/b2
This issue has been automatically marked as "transition" to indicate the potential for needing transition to the new B2 development project.