set proper shared lib soname on Linux/Unix

 * Set a correct shared lib soname on Linux or other Unix systems (except OSX and Solaris) that ldconfig can work with. The name will have the pattern `liblsmash-$(VERSION).so.$(API)`. API is always set to 0, it must be explicitly set to something else in config.mak or configure if that's required. Will close https://github.com/l-smash/l-smash/issues/27
 * Script was reworked to be compatible with POSIX shell, so Bash is not a requirement (useful on non-GNU systems).
 * I fixed the detection of whitespaces in the source path, which didn't work properly.