Bump CMake requirement to 2.8.8

Permits use of CMake "object libraries," and CMake-based detection of
compiler ID and version. Avoids bug in add_library(GLOBAL). Various
minor CMake logic simplifications ensue.

2.8.8 will be 21 months old by the time we release GROMACS 5. Anybody
who installed the latest CMake since we released 4.6 betas already has
a suitable version. Most distros already supply a higher version, with
the notable exception of Ubuntu 12.04 LTS (supplies 2.8.7). A new
Ubuntu LTS is due shortly after GROMACS 5.0, and it will have a
suitable CMake.

Also moved the code supporting storing the compiler info in
buildinfo.h to where it is first used, instead of some point in
top-level CMake scope.

Fixes #1287

Change-Id: I7b82412d2b51a7a0b2e94022e8e39cf3ae06cc35