4.2.4. Separate builds

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

This may be enabled by setting the make macros SEPARATE_BUILDS, SEPARATE_COMPILERS, and SEPARATE_CPUS, e.g. in an options.mkf file, placed either in $(TOP) or locally.

The file could contain, for example:

CPU=avx2
COMPILER=ifc
SEPARATE_BUILDS=y
SEPARATE_COMPILERS=y