Revert "Compatibility fix with automake 1.14"

This reverts commit 7541bb52f5bcf99615c34670e5febd04666662b7.

This commit was breaking compilation of MEX files: the subdir-objects option
does not deal well with source files that are under a distinct tree.
No longer consider automake warnings as fatal

With automake 1.14 there are a lot of new warnings related to the way we deal
with MEX files (they are compiled from source files located in a different
tree). Automake wants us to use the 'subdir-objects' option, but it breaks in
our case.

In automake 2.0, the 'subdir-objects' option will be activated by default, so
we will have to refactor the build system for MEX files. But for now we can
live with the automake warnings.