* These variables are checked via the JSON files in PIO control setups
  and should not change type unless necessary. Consistency is not a
  necessity.

At least Automake 1.16.1 does not seem to have problems with this.

This partially reverts "Account for the case when YAXT and PPM are provided as '/path/to/lib/libname.a'."

This enables building with versions of libtool that are patched to avoid
overlinking (e.g. on Debian systems). It is also a bad idea in general to rely
on negative side-effects, which overlinking is, and we try to avoid that.

This is similary to what we do for the parallel cksum tests.

This reverts commit 4c120cc6.

This makes the branch identical to 2a4af8e6.

This way, we avoid target-specific flags '-DUSE_MPI', which result into
target-specific rules that do not use Libtool for the compilation.