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.

 #define LEMON_VERSION "@PROJECT_VERSION@"

 #cmakedefine LEMON_HAVE_LONG_LONG 1

 #cmakedefine LEMON_HAVE_LP 1

 #cmakedefine LEMON_HAVE_MIP 1

 #cmakedefine LEMON_HAVE_GLPK 1

 #cmakedefine LEMON_HAVE_CPLEX 1

 #cmakedefine LEMON_HAVE_CLP 1

 #cmakedefine LEMON_HAVE_CBC 1