GNUmakefile: add support for LIBSSH2_LDFLAG_EXTRAS
GNUmakefile: add support for LIBSSH2_LDFLAG_EXTRAS

It is similar to existing LIBSSH2_CFLAG_EXTRAS, but for
extra linker options.

Also delete some line/file ending whitespace.

closes #27

libssh2/libssh2,libssh2/libssh2,libssh2/libssh2,libssh2/libssh2