Fix a misunderstanding that's been present since the inception. The
LDFLAGS variable is entirely at the builder's disposal. Flags needed
for system-internal settings must go in the AM_* equivalents.
When OPM-Core is used as a Dune module (e.g., as enabled by commit
789bc5ca7), we stand the risk of creating multiple, conflicting
defintions of crucial <config.h> symbols (e.g. `HAVE_BOOST') and
build variables (e.g., `BOOST_CPPFLAGS' and `BOOST_LDFLAG').
Avoid this situation by prefixing these symbols with an `OPM_'
string such that Dune's `ENABLE_BOOST' solution works as intendend
in modules that are derived in the OPM context.
