Set the version to 'hg-tip' if everything fails

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.

Put the version string into config.h

Also make it possible to set the version using the LEMON_VERSION environment
variable, and use the hg revision as version string in case this variable is
unset.