Findewoms.cmake: fix typo

it's 'start.hh', not 'start.h'...
Use dSYM bundles as debugging containers (on MacOS X)

gdb on MacOS X does not seem to be very fond of "flat" dSYM files, but
it can read the bundle variant fine. Thus we change that to enable
debugging across several tools.
Merge pull request #24 from bska/024_cmake

Synchronise build system with opm-core to restore debugging on MacOS X