Added flag to support C99 standard and reorganized the Makefile

Reorganized makefile & added relevant flag in support of C99 standard. However, this will require to run the make commands (e.g. make clean) with root privileges.
Using make all for building all the targets if good. Unfortunately, it is more tricky than running sudo make all.

For correct building, environment variables are needed such as $TARGET and $PATH. So one should run sudo -E <comand> instead.

However IMHO, this may have some drawbacks: (I) bypassing the $PATH environment variable may require appropriate changes on suders file; and (II) it is not that obvious for novices, who may give away Nanvix due to difficulties on simply building it.

Nevertheless, using all to build nanvix and documentation targets is a good strategy.