Build osmium_convert in order to be able to eventually replace osmosis

This provides a script to build the libosmium examples such as
osmium_convert (to convert from xml to pbf).

I initially tried a CMake ExternalProject setup, but this was more
complicated than I initially thought; this is the more elegant solution.

The goal is to eventually rip out osmosis, so that we no longer depend
on java for the cucumber tests.

References:

- https://github.com/Project-OSRM/osrm-backend/issues/1788

.gitignore

@@ -41,6 +41,7 @@ Thumbs.db
 #######################
 /build/
 /cmake/postinst
+/third_party/libosmium/tmp/
 
 # Eclipse related files #
 #########################