update git URL to point to the up-to-date repo...
only run additional checks if feature is not disabled

GEM_CHECK_LIB should not not run AC_CHECK_...  if it has been
explicitely disabled via "--without-..."

Closes https://github.com/umlaeute/Gem/issues/25
removed "force" option from GEM_ARG_(DIS|EN)ABLE

even I couldn't remember what this is needed for...

Closes: https://github.com/umlaeute/Gem/issues/19