Linked build of GDAL against libkml

[thclark]

Work in progress against #2

This PR:

• [x] installs libkml-dev in the build environment so that GDAL can perform its compile-time checks and link against it
• [x] links the gdal build against libkml
• [x] limits GDAL build to shared-only per the python buildpack behaviour (building both would cause a lot of apps to fly over violate heroku's soft slug size limit, I believe... the static lib is c.170mb as I recall)
• [ ] installs libkml-dev in the slug so GDAL can find it at runtime

Checks required:

• [ ] Heroku staff to check that removing static libs really is a good idea (i.e. was it intentional that you added static back in?)

Help needed from @CaseyFaist or @KevinBrolly, if you don't mind? I've made an attempt to apt-get install libkml-dev in the bin/compile script, but it's not working. I get: E: Unable to locate package libkml-dev, which is weird because it's in the ubuntu universe repository.

I'm uninclined to use heroku-apt-buildpack to install it, because that'd mean you need two buildpacks rather than one; and I'm not at all familiar with the way heroku compiles and deploys slugs so not sure I have the library configured correctly either.

Any takers for help teasing out that last issue?

[thclark]

Closing, since @KevinBrolly has solved in an alternative way (see conversation in #2 )