pkg-config: optimizations
pkg-config: optimizations

 * cache the presence/absence of pkg-config package definitions.
   The PkgConfig.clear_cache method allows to clear the cache
 * load the package version only when requested

Those changes reduced the loading time for a full-blown Orocos/Roby
controller from 9 seconds to 3 :)

orocos-toolchain/utilrb-ext,orocos-toolchain/utilrb-ext