Delete cache variables relative to C and CXX compilers in distcheck target

  distcheck target copies CmakeCache.txt file and removed two entries.
  With package using catkin, the distcheck target failed since for some reason
  cmake did not source again cmake files checking for C and CXX compilers.

  As a result, CMAKE_LIBRARY_ARCHITECTURE variable is not set and find_package
  macro of roscpp fails to locate libpthread.so because it does not look in
  /usr/lib/i386-linux-gnu (for Ubunutu 12.04 32bits).

  Deleting cache variables relative to the compiler force distcheck target to
  source again the appropriate cmake files.