Fix links to build directory.

When switching to another branch. The links (flow123d, mpiexec, and unit_test makefiles)
become invalid. This patch:

Introduce only one link 'build_tree' to the built tree. Other links points only through this one,
so if we change 'build_tree' other links may be OK.

Add bin/git_post_checkout_hook which also serves as check script for correctness of build_tree link.

Add call of the git_post_checkout_hook to:
CMakeLists.txt, makefile (get build dir from the link), tests/all_tests.mk, make some targets dependent on
a new check target.

minor changes in flow123d.sh and Flow123d_git_rev_num.cmake
Merge branch 'master' of github.com:flow123d/flow123d

Conflicts:
	makefile