install-libssh2.sh: Support Mac OS X (via Mac Ports)

Make it possible to install /usr/local/lib/libssh2.a under Mac OS X,
when MacPorts is installed.
install-libssh2.sh: Use curl when wget is not installed

When wget is not available, use curl:
Mac OS X 10.9 seems to have curl in /usr/bin/curl
Merge pull request #4 from tboegi/install_libssh2_for_Mac_OS_X

Install libssh2 for mac os x