3.3.4. Separate builds

Especially when developing, it saves time to have separate build_*/ directories; one for each combination of SOLVER and OPTS.

This may be enabled by setting the make macro SEPARATE_BUILDS, e.g. in a $(TOP)/options.mkf file (which might also be used to set the COMPILER and other macros). The file could thus contain, for example:

COMPILER=ifort
SEPARATE_BUILDS=on