Add pkg-config based dependencies.
Merge pull request #12 from pmienk/master

Add pkg-config based dependencies to autotools gsl
Problem: Android needs to provide special LDFLAGS to libtool linking
Solution: Accommodate the LIBTOOL_EXTRA_LDFLAGS env var
Merge pull request #13 from jemc/master

Problem: Android needs to provide special LDFLAGS to libtool linking
Update README.md

We don't generate a prelude header.
Problem: projects should not be generating prelude header

Solution: stop generating this (it comes from CZMQ)

Note: in CZMQ project.xml, czmq_prelude.h will have to be defined as
a public header file.
Merge pull request #16 from hintjens/master

Problem: projects should not be generating prelude header
Problem: need to signal to zproto when using generated header

Otherwise zproto regenerates typedefs for opaque types, in message
and client codecs.

Solution: define macro when defining typedefs. Needs matching change
in zproto.
Merge pull request #17 from hintjens/master

Problem: need to signal to zproto when using generated header
Problem: it's clumsy to specify dependencies

I don't like having to expose autoconf semantics in the project model.
(This also applies to package_dependency, which still needs fixing).
The project model should be a clean abstraction. If e.g. Zyre depends
on CZMQ, then it should say just that, no more. It's up to zproject
to do the rest.

Solution: build knowledge of standard projects into zproject, so the
project.xml can say,

    <use project = "czmq" />

And the rest happens automatically.

- known projects are zyre, czmq, zmq
- easy to add more to class_projects.gsl
- resolves further dependencies automagically (zyre->czmq->zmq)
Merge pull request #19 from hintjens/master

Problem: it's clumsy to specify dependencies
Problem: Does not add $(project.name).h to automake
Solution: Add it unless project.name matches a class.name
Merge pull request #20 from jemc/master

Problem: Does not add $(project.name).h to automake
Problem: Other build systems need header fix from PR #20
Solution: Add header to other build systems
Merge pull request #21 from jemc/master

Problem: Other build systems need header fix from PR #20
Problem: Need doc generation and PR #15 has merge conflicts
Solution: Merge as a patch by hand (with a few tweaks)
Merge pull request #22 from jemc/master

Problem: Need doc generation and PR #15 has merge conflicts
Problem: project license used in multiple gsl files
Solution: resolve licene.xml in project.gsl instead of build-class.gsl
Merge pull request #23 from sappo/master

Problem: project license used in multiple gsl files
Problem: Only one dependency level will be resolved.
Solution: Do recursion in class_projects.gsl