[dynlink] require dynlink support; force plug-ins to be dynamic

Makefile

Makefile.generating

configure.in

doc/userman/user-plugins.tex

doc/userman/user-start.tex

headers/header_spec.txt

ptests/ptests.ml

share/Makefile.config.in

share/Makefile.dynamic

share/Makefile.plugin.template

src/libraries/stdlib/FCDynlink.ml

src/libraries/stdlib/dynlink_native_ko.ml

src/libraries/stdlib/dynlink_no_native.ml

tests/misc/bts1347.i

tests/misc/ensures.i

tests/misc/oracle/bts1347.res.oracle

tests/misc/oracle/ensures.res.oracle

tests/misc/remove_status_hyps.i

tests/saveload/load_one.i

tests/saveload/oracle/load_one.res.oracle