support alternative location for build intermediate

Currently build intermediates (eg, various .dep and .o files) are split out into a separate directory. For example,

project
|-- applications
|-- src
|-- build
|   \-- linux64Gcc...
|-- platforms
|    \-- linux64Gcc...

This generally works well for intermediates (build) separated from final (platforms), however if we wish to rebuild portions of the code is can be a slight problem if the entire volume is readonly.

If the OpenFOAM directory is readonly, then we cannot writing into the project directory, but perhaps not into the source Make/ subdirectory either. For these cases (or maybe if we want to have the build intermediates land on faster disk), support FOAM_BUILDROOT and/or wmake -build-root=....

Already made similar local changes to ThirdParty.