Build manuals when ./configure --enable-manuals is specified. Requires osmo-gsm-manuals to be installed. Related: OS#3385 Depends: I96933dd4dd6cac159847647f1c642215051a9aad (osmo-gsm-manuals) Change-Id: I2f9c27f81940b02414f0e618483bffade72e8cf7