Revert flag environment variable semantics.

Now environment variables will be "tried" to be applied as flags but
if no flag is known given an environment variable then the loader will
not fail.

Review: https://reviews.apache.org/r/13173
Added MESOS_NATIVE_LIBRARY to launcher.

Review: https://reviews.apache.org/r/13174
Set MESOS_NATIVE_LIBRARY in the testing environment.

We want to force our tests (including the Java ones) to use the most
recent version of our library, so we set MESOS_NATIVE_LIBRARY.

Review: https://reviews.apache.org/r/13175
Cleaned up the pre-installed "wrappers".

Review: https://reviews.apache.org/r/13176
Add '-fno-strict-aliasing' for gcc 4.4.*.

Review: https://reviews.apache.org/r/13182
Updated configure.ac to properly check for Python.h.

Review: https://reviews.apache.org/r/13194