Add new environment variables WITH_MANUALS and PUBLISH to control if
the manuals should be built and uploaded. Describe all environment vars
on top of the file.
When WITH_MANUALS is set, install osmo-gsm-manuals like any other
dependency and add --enable-manuals to the configure flags (for "make"
and "make distcheck"). Add the bin subdir of the installed files to
PATH, so osmo-gsm-manuals-check-depends can be used by ./configure.
Related: OS#3385
Change-Id: I4cf9d3c21f3912eac3c51bae1ac7b2ad0845c947
See osmo-ci change I2409b2928b4d7ebbd6c005097d4ad7337307dd93 for rationale.
Depends: I2409b2928b4d7ebbd6c005097d4ad7337307dd93
Change-Id: I7e9f19755f55b274def9d757f72f2a08a01a4cce