Minor CMake improvements

* Use the empty ELSE(), ENDIF(), ENDMACRO(), etc. syntax since this improves
readability and requires less typing.
* Use FIND_PACKAGE() instead of INCLUDE().
* Use the value of CMAKE_COMMAND variable instead of cmake, since this works
even if cmake is not in the PATH.

 SET(LEMON_VERSION "@PACKAGE_VERSION@" CACHE STRING "LEMON version string.")