[Makefile] Do not explicitly add gmp to the .merlin file.

Instead, plugins requirements should be automatically added to the .merlin file.

Makefile.generating

@@ -195,7 +195,6 @@ endif
 	for PKG in $(LIBRARY_NAMES); do echo PKG $$PKG >> .merlin; done
 	for PKG in $(LIBRARY_NAMES_GUI); do echo PKG $$PKG >> .merlin; done
 	for PKG in $(MERLIN_PACKAGES); do echo PKG $$PKG >> .merlin; done
-	echo "PKG gmp" >> .merlin
 	echo "B lib/plugins" >> .merlin
 	echo "B lib/plugins/gui" >> .merlin
 	find src \( -name '.*' -o -name tests -o -name doc -o -name '*.cache' \) -prune \