Change conflicting JVM features from warning to error

If configure is given a conflicting configure line, e.g. --disable-jvm-feature-serialgc --with-jvm-features=serialgc , it will print a warning and let the disable override the enable.

This turned out to be not enough to catch incorrect configurations. Instead, configure should consider this an error, and ask the user to make up their mind.

Note that this only can happen if you mix the old-style --with-jvm-features=[-]* with the new-style --enable/disable-jvm-feature-*. Autoconf will let later --enable/disable flags override previous enable/disable, and likewise with --with flags.