It seems that build does not check the Roundcube version, only the MySQL version:

Code:
if ${OLD_MYSQL}; then
        echo "${boldon}RoundCube ${ROUNDCUBE_VER} requires newer version of ${MYSQLNAME} than ${MYSQLV}, alternatively, the following my.cnf configuration can be set:${bold$
        echo "innodb_large_prefix=1"
        echo "innodb_file_per_table=1"
        echo "innodb_file_format=Barracuda"
        exit 1
fi

Workaround is replacing the check in the build script:

Code:
cd /usr/local/directadmin/custombuild
cp -a build build.bak
sed 's/if \${OLD_MYSQL}\; then/if \${OLD_MYSQL} \&\& \[ "${ROUNDCUBE_VER}" -ge "1.5.0" \]\; then/' -i build

It should look like this:

Code:
if ${OLD_MYSQL} && [ "${ROUNDCUBE_VER}" -ge "1.5.0" ]; then

After the change it is possible to install or downgrade to Roundcube 1.4.x.