cmake: enable shared libraries by default on Hurd

Since the toolchain is mostly the same as used on e.g. GNU/Linux
(glibc, gcc, binutils, etc), they can be safely enabled.

Change-Id: Ic663c8c344b5dfa2910e9c30260588e1917038c1
Fix clang 3.5 warnings regarding *abs*

Fix abs() family functions type usage mismatch.

Change-Id: I85ed2931d681aa1ad024678b4209a524abc2cc61
Signed-off-by: Alexey Shvetsov <alexxy@omrb.pnpi.spb.ru>
Merge release-4-6 into release-5-0

Conflicts:
CMakeLists.txt (applied to cmake/gmxManageSharedLibraries.cmake)

Replaced gmx_llabs by llabs as mentioned in TODO.

Change-Id: I37e8cf5d6725cc9836e6dc29ca44e7ef915417c4