[DOFMapper] Shut up clang warning about having to specify return type of a multi-statement lambda
[EntityWrapper] Make EntityPointer typedef in SkeletonIntersectionWrapper public

The typedef for the EntityPointer in SkeletonIntersectionWrapper should be
public, just like the equivalent typedef in the IntersectionGeometry class
from plain PDELab.
[Buildsystem] Replace obsolete header AM_CONFIG_HEADER with AC_CONFIG_HEADERS