Fix warnings from compiler.

userinfo is not a pointer-to-pointer, but just a simple pointer.

recvfrom() accepts a generic pointer, rather than a char one, and
requires a socklen_t argument, not an int one.
doc: don't use the full path to the module in the usage examples.

The path depends on the bitness of the software, so it can't be used
this way.
build: use GCC for build, and ensure that unneeded symbols are not exported.
build: do not force flags, add to them instead.