pingus 0.7.2: fix newer boost(1.45.0) detection (break older boosts)

pingus 0.7.2: fix newer boost(1.45.0) detection (break older boosts)

Without that fix we have:
  Fatal Errors:
    * library 'boost_signals' not found

  FATAL: scons build execution failed.

Note that this commit is incompatible with older(than 1.45.0) boost.

From #openmoko-cdevel on Freenode on irc:
  Feb 12 11:43:07 <JaMa|Off>      GNUtoo|laptop: that boost version is default,
  so if you fix it for default version and break for older it's not so bad (
  distros using older boost should handle it)
  [...]
  Feb 12 11:48:37 <GNUtoo|laptop> JaMa|Off, can I fix like that or should I send
  a patch for breaking older boosts and distros?
  Feb 12 11:49:35 <JaMa|Off>      GNUtoo|laptop: nobody in OE is pinning boost..
  so just note it in commit (that it won't work with old boost anymore)

The patch is kept in tree because it could be usefull if:
 * Someone is still using old boost
 * -mt versions are re-added in the future

Signed-off-by: Denis 'GNUtoo' Carikli <GNUtoo@no-log.org>