build: rename --enable-shared switch

Rename it to --enable-libmpv-shared. The option name didn't really
tell much. When we add the possibility to create a static library,
it would also be bad if that were named --enable-static (because it
would sound like it does what --static-build does).
build: automagically extract client library version

This reads MPV_CLIENT_API_VERSION from the source header, and turns it
into a 3 part version number.

E.g. if MPV_CLIENT_API_VERSION were 0x12abcdef, this would result in
"18.171.773615" (8 bits, 8 bits, 16 bits).

We'll see if this is actually useful, or if it's too clever.
stream_file: network file system detection for Linux

Addresses issue #558 on Linux systems.

Signed-off-by: wm4 <wm4@nowhere>
build: make check for BSD fstatfs() a bit more strict

Linux also has fstatfs(), and the test relied on certain system include
files being available on BSD, but not on Linux. It would break if Linux
added the missing includes for some reason.

Make it a bit stricter, and check for the struct statfs field the code
needs.