Cast MUMPS data int indices to dolfin::la_index. Caused failure of compilation with 64-bit indices in PETSc.
Check that GCC version is at least 4.6.

4.6 is reported to work, and 4.4 is know to fail. Not sure about 4.5, but it's pretty old anyway.
Add function SLEPcEigenSolver::eps() to access pointer to unerlying SLEPc object.
Ignore SLEPcEigenSolver::eps() in SWIG interface
Print GCC version if version check throws error.
Remove instructions for 'in-tree' builds (fixes issue #429).
Change custom target for building tests from 'test' to 'tests' (fixes issue #428).
Only check gcc version with more rececnt CMake that provides compiler version varialble.

Maybe not so useful becasue more recent CMake probably also means more recent gcc.
Merge branch 'garth/fix-issue-408-check-gcc-version'
Merge branch 'garth/fix-issue-407-access-eps-pointer'
Merge branch 'jan/fix-mumpslusolver-int'