C++11: Update compilation rules to specify C++11 support and conformance

The change from C++0x to C++11 allows all of C++11 functionality to be
used in OpenFOAM, in particular constructor delegation which avoids code
duplication or constructor helper functions.  However, this also means a
change to the minimum gcc version supported which is now 4.7 rather than
4.5.

Note that gcc-4.7 does not support the entire C++11 standard but does
support all of the functionality currently needed for further OpenFOAM
development.  The minimum gcc-version which supports the entire C++11
standard is 4.8 which is now the recommended minimum gcc version.