add is_required function, use it when verifying toolchain definition vs toolchain module; this allows to make BLACS optional
filter ScaLAPACK dep LIB* variables based on whether they are set or not (required to filter out LIBBLACS)
rebase ScaLAPACK on top of BLACS, define is_required to exclude BLACS with ScaLAPACK 2.x, only set BLACS variables if required
only include ScaLAPACK in toolchain definitions, BLACS is included indirectly; don't set BLACS variables empty, Blacs support takes care of proper handling of that
Merge pull request #638 from boegel/optional_blacs

make BLACS optional as toolchain element, depending on ScaLAPACK version