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>
Prohibit AVX_256 with buggy gcc 4.6.1

Fixes #1259

Change-Id: I7c523a90ccd0dd8fd3df6cb1914429e22a27ab5d
Fix perturbed energy calculation in wall.c

For the perturbed energy, the state A is (1-lambda) whereas B was set to
0 and correspondingly the wall interaction for type B is switched off.

The correct expression for the B-state should be lamfac = lambda.
Merge remote-tracking branch 'gromacs/release-4-6' into release-4-6_ASFeature
Merge remote-tracking branch 'origin/release-4-6-lambdaWallFix' into release-4-6_ASFeature
Fix conditional in tpi.c

z=0 and z=L_z are valid parameters for the insertion coordinate