Define `OPM_HAVE_BOOST' to zero if unavailable.

This way, there will be no compiler warnings if the symbol is used
in a way akin to

   #if OPM_HAVE_BOOST
      /* Boost is available */
   #else
      /* Boost is not available */
   #endif

m4/opm_boost_base.m4

@@ -243,6 +243,8 @@ if test "x$want_boost" = "xyes"; then
         fi
         # execute ACTION-IF-NOT-FOUND (if present):
         ifelse([$3], , :, [$3])
+        AC_DEFINE([OPM_HAVE_BOOST], [0],dnl
+                  [Define to `0' if the Boost library is not available])
     else
         AC_SUBST([OPM_BOOST_CPPFLAGS])
         AC_SUBST([OPM_BOOST_LDFLAGS])