Fix another fall-out from Boost library clean-up

We failed to propagate the BOOST_\(.*\)FLAGS -> OPM_BOOST_\1FLAGS fix
(commit 6b57a00) to all clients, notably our own macro for recognising
whether or not the Boost.Test library should be linked dynamically or
statically.  This commit remedies that oversight.

m4/opm_dynlink_boost_test.m4

@@ -44,8 +44,8 @@ AC_REQUIRE([AX_BOOST_UNIT_TEST_FRAMEWORK])
 _opm_LIBS_SAVE="${LIBS}"
 _opm_CPPFLAGS_SAVE="${CPPFLAGS}"
 
-LIBS="${BOOST_LDFLAGS} ${BOOST_UNIT_TEST_FRAMEWORK_LIB} ${LIBS}"
-CPPFLAGS="${BOOST_CPPFLAGS} ${CPPFLAGS}"
+LIBS="${OPM_BOOST_LDFLAGS} ${BOOST_UNIT_TEST_FRAMEWORK_LIB} ${LIBS}"
+CPPFLAGS="${OPM_BOOST_CPPFLAGS} ${CPPFLAGS}"
 
 AC_LANG_PUSH([C++])