FindAlugrid.cmake: do not try to include the whole grid

first this makes the test very slow, second (and more importantly) it
makes the test fail on dune master as the header relies on the
presence of the DUNE_GRID_VERSION* macros which are not present at the
configure stage of the build...
Finddune-grid.cmake: always enable the experimental grid extensions

this is now done by a config.h macro. The reason is that quite a bit
of code depends on boundary IDs which are only available as an
"experimental" grid extension in recent Dune releases...
build system: do not separate the debugging symbols by default

the reason is that the tools for profiling and for coverage analysis
(i.e., gprof and gcov) have trouble with this and crash. if you want
this to be enabled for whatever reason, use the cmake flag
-DSTRIP_DEBUGGING_SYMBOLS=ON or pass configure the
--enable-strip-debug option.