further Makefile improvements

* use $@ in Makefile.dep and clean rules
* delete Makefile.dep, gtest, ... when cleaning
* remove FORCE target, use .PHONY
* remove some unused variables
Merge pull request #59 from Waldteufel/makefiles

further Makefile improvements
MATH: New small algorithm: sort_values for three values
MVE: Added mve::TriangleMesh::duplicate() function
MVE: Added mve::geom::mesh_merge function, mve::geom::mesh_components function
Merge branch 'master' of https://github.com/simonfuhrmann/mve
MVE: Quick fix for writing .bnpts files
UTIL: Function declaration fix
Merge branch 'master' of github.com:simonfuhrmann/mve
MVE: Updated TriangleMesh and VertexInfo
Merge branch 'master' of github.com:simonfuhrmann/mve
MVE: Added TriangleMesh::delete_invalid_faces() with tests