Warn (or error with explicit --with-libgmp) if gmp is not actually found

configure.ac

@@ -427,6 +427,11 @@ if test "x$have_openssl" != "xyes"; then
     AC_SEARCH_LIBS([__gmpz_init], [gmp], [have_libgmp=yes], [have_libgmp=no])
     if test "x$have_libgmp" = "xyes"; then
       AC_DEFINE([HAVE_LIBGMP], [1], [Define to 1 if you have libgmp.])
+    else
+      AC_MSG_WARN([libgmp not found])
+      if test "x$with_libgmp_requested" = "xyes"; then
+        ARIA2_DEP_NOT_MET([libgmp])
+      fi
     fi
   fi
   if test "x$with_libgcrypt" = "xyes" &&