Noel, I don't think you want to use GPL, as this is how I understand it. If you use GPL it means you are forcing the author of the work that uses your library to provide the source code, if so asked. You would also, from the terms in the license, basically be forcing the author to GPL his work too. The author must provide all of the source, not just the source to your library. from the license...
"For an executable work, complete source code means all the source code for all modules it contains, plus any associated interface definition files, plus the scripts used to control compilation and installation of the executable."
This is really bad because some of the code may be under other licenses but the author is still required by the GPL to give it out. Now there are a couple things that can be done to help here because you can't break the license you had with the other authors, but it's just way too much trouble and complicated.
I don't think you want to use LGPL either because of this line...
"As an exception to the Sections above, you may also compile or link a "work that uses the Library" with the Library to produce a work containing portions of the Library, and distribute that work under terms of your choice, provided that the terms permit modification of the work for the customer's own use and reverse engineering for debugging such modifications."
LGPL works best for dynamically linked libraries and would perform the way I think you want if your library was a dynamically linked one.
from reading your post, I think you want Mozilla Public License (MozPL or MPL)
"The MPL, authored by Netscape Communications as part of their open-source release of Communicator 5, strikes a balance between the BSD license and the GPL. Private derivative works are permitted, while changes to MPL-covered source must be made freely available on the Internet. The MPL, however, is non-viral: additions to (as opposed to modifications of) the MPL-licensed source which form a "larger work" may be licensed differently and need not be published at all."
I have not yet gone through the whole license myself.
"For an executable work, complete source code means all the source code for all modules it contains, plus any associated interface definition files, plus the scripts used to control compilation and installation of the executable."
This is really bad because some of the code may be under other licenses but the author is still required by the GPL to give it out. Now there are a couple things that can be done to help here because you can't break the license you had with the other authors, but it's just way too much trouble and complicated.
I don't think you want to use LGPL either because of this line...
"As an exception to the Sections above, you may also compile or link a "work that uses the Library" with the Library to produce a work containing portions of the Library, and distribute that work under terms of your choice, provided that the terms permit modification of the work for the customer's own use and reverse engineering for debugging such modifications."
LGPL works best for dynamically linked libraries and would perform the way I think you want if your library was a dynamically linked one.
from reading your post, I think you want Mozilla Public License (MozPL or MPL)
"The MPL, authored by Netscape Communications as part of their open-source release of Communicator 5, strikes a balance between the BSD license and the GPL. Private derivative works are permitted, while changes to MPL-covered source must be made freely available on the Internet. The MPL, however, is non-viral: additions to (as opposed to modifications of) the MPL-licensed source which form a "larger work" may be licensed differently and need not be published at all."
I have not yet gone through the whole license myself.