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.