menuselect: Remove ineffective weak attribute detection.

menuselect detects compiler support for multiple styles of weak
functions.  This is a remnant from 2013 when OPTIONAL_API required weak
functions.  It is no longer correct for menuselect to switch
dependencies from optional to required based on lack of weak function
support.

Note an issue remains - dependencies should switch from optional to
required based on OPTIONAL_API being enabled or disabled.  I don't think
this is possible.  menuselect needs to know at startup if OPTIONAL_API
is enabled or disabled, so the only way to fix this is to remove
OPTIONAL_API from menuselect and create a configure option.  I've left
the code that switches in place but it's preprocessed out.

Additionally removed:
- WEAKREF variable from Asterisk makeopts.in.
- Related disabled code from test_utils.
- Pointless AC_REVISION call from menuselect/configure.ac.

Change-Id: Ifa702e5f98eb45f338b2f131a93354632a8fb389