[duneproject] fix section name in generation of config.h.cmake

When creating a project and generating the config.h.cmake file, duneproject
surrounds the inserted text with {begin,end} $NAME. $NAME is set to $PROJECT
without a leading "dune[-_]" and $PROJECT is the module name specified by the
user (e.g. $PROJECT==dune-grid, $NAME==grid).
When DuneMacro.cmake generates the config.h.cmake for the created project in
the build-directory, it searches the config.h.cmake file in the source
directory of the project for {begin,end} $ProjectName which is set to
DUNE_MOD_NAME, which is the module name specified in the corresponding
dune.module file (e.g. dune-grid).
This leads to DuneMacro not finding and therefore ignoring the block in the
source config.h.cmake of the new project.

(cherry picked from commit f9e3fbf67b573f355ac2222431f352fd5788179c)
Signed-off-by: Christoph Grüninger <gruenich@dune-project.org>