Avoid failing build for warning in an integration context.

[krocard]

Forbidding warnings (Werror) when modifying the PF is beneficial for code quality. Nevertheless when integrating the PF in an other component, one should not have to modify the FP code to compile it if there is only warnings.

Proposition: Add a PF option "FAIL_BUILD_ON_WARNING" (or maybe "PF_DEVELOPEMENT" ?) default to true (to make sure external contributers do not introduce warnings) that enables Werrors flags.

[krocard]

Should be fixed by #310.

[dawagner]

Done.