doc/manuals: support OSMO_GSM_MANUALS_DIR

Instead of only relying on pkg-config, support the environment variable
and fallback path like in other Osmocom repositories too. The same
script is used in other Osmocom repositories where manuals are built
without autotools.

Related: OS#4912
Change-Id: Iecfbfc2a7588c316642aa37be637cf3c02f76ac4
2 files changed