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

The idea is that when an additional feature is enabled, we should run the same
tests as without it plus additional tests for the feature.

This is the first step towards building serial versions of tests also when MPI is enabled.

This is the first step towards switching to the parallel test harness.