[Doc] Add missing autogen to example builds

[AmirAbrams]

The Windows cross-compile build examples are missing the autogen.sh step. Without that step, configure doesn't exist and you get this error:

bash: ./configure: No such file or directory

Using Ubuntu 16.04 64 bit.

[laanwj]

ACK.

Using Ubuntu 16.04 64 bit.

If you are cross-compiling from Ubuntu 16.04 I'd suggest reading #8732 #8653 #8730 #8511, there are serious current compatibility issues.

[luke-jr]

configure comes with the source code packages. You should not be using git unless you are a developer.

[laanwj]

But in practice most people following this guide seem to be cloning from git. No one uses the source tarball. It's mostly developers that follow this guide in the first place. Also IIRC we mention autogen.sh in all other places (such as build-unix.md).

It could ofc. be added that it is optional.

[AmirAbrams]

@luke-jr I always build from git but I am a developer and thought the document was intended for developers. If not, then please close my PR but like @laanwj points out, build-osx.md and build-unix.md reference the autogen.sh step. Maybe I can add an if/fi that checks for the configure file or explain the example is for source code packages?

[laanwj]

Maybe I can add an if/fi that checks for the configure file or explain the example is for source code packages?

Maybe just add a comment after it # not required when building from tarball. Adding a fi/if construction sounds like overkill, and anyhow this is based on information only the user can know.

[maflcko]

Agree with @laanwj

[AmirAbrams]

Is it okay like this?

cd ..
./autogen.sh # not required when building from tarball
./configure --prefix=`pwd`/depends/i686-w64-mingw32

If you agree, I will make the change and squash into one commit.

[AmirAbrams]

I made the corrections and squashed my commits.