Add a small bootstrapper to locate build system

Currently this bootstrapper only finds the macros that are located
in the cmake/ tree in this project. Projects that don't carry their
own build system can specify search locations here.
Allow source location to be overridden

By default the configure script assumes that the source is located
relative to the script itself. By allowing this to be overridden, the
script can be shared by other projects, each passing their own location
as an extra parameter.
Search for directory containing build scripts

Currently only the source tree is "searched", but the template could
be expanded to look in another/more locations (or be specified
explicitly on the command-line with a --with-opm-macros= option)