fixed: ERT depends on libdl

this issue was triggered with static libs for ERT
ERT: link to the threads library of the OS

this is required to make the opm-core build succeed if ERT was build
with -DBUILD_SHARED_LIBS=OFF . (without it, I get errors like

    /home/and/src/ert/devel/libert_util/src/thread_pool_posix.c:328: error: undefined reference to 'pthread_create'
Add refDepthDefaulted field to Well class.
Adapt tests to use new Well constructor, add test for defaulted ref depth.
Change Well constructors to avoid explicit bool arg.

Suggested by Joakim Hove.
Update checkWELSPECSConsistency() method.

Now accounts for new refDepthDefaulted() method and semantics.
Merge pull request #274 from atgeirr/default-wellrefdepth

Correctly handle defaulted well reference depths
Merge pull request #273 from andlaus/allow_static_ERT

Allow static ert
add the FGIT keyword to the FIELD_PROBE

This is used by Norne, but for some reason it was not included in the
deck_names list...