[Merge][Bugfix] r7422 from branches/release-cmake-2.2: Prevent definition of HAVE_BOOST without value due to race conditions i
n some autoconf versions.

For some autoconf versions there was a race condition between ax_boost_base.m4 a
nd dune_boost_base.m4 resulting in HAVE_BOOST being defined with no value (inste
ad of ENABLE_BOOST). For others there were even two definitions of it (one with 
and one without value). 

This patch renames the symbol being defined in dune_boost_base.m4 to HAVE_DUNE_B
OOST and defines a value for it if boost is found.


[[Imported from SVN: r7424]]