Steps to reproduce:

Install latest libssh (0.11). Run configure:

$ ./configure --enable-server --enable-agent --enable-proxy \
    --enable-ipv6 --with-mysql --with-ssh=/home/alex/libssh ...

Result:

configure: error: SSH library version requirement not met (>= 0.6.0)

Expected:

Successful configuration.

Analysis:

Some libssh versions declare LIBSSH_VERSION_MAJOR/LIBSSH_VERSION_MINOR in libssh.h whereas others in libssh_version.h. libssh.m4 checks both places for includes in /usr/include but doesn't do the same in a custom path.