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.