[configure] Support for flambda flags.

We add a new option to configure `-flambda-opts` to allow passing
custom flags to flambda. Example:

```
./configure -flambda-opts "-O3 -unbox-closures"
```

1 files changed, 18 insertions, 0 deletions

diff --git a/INSTALL b/INSTALL
index 676a1f8ea..808e76882 100644
--- a/INSTALL
+++ b/INSTALL
@@ -128,6 +128,24 @@ INSTALLATION PROCEDURE IN DETAILS (NORMAL USERS).
 	Use <command> to open an URL in a browser. %s must appear in <command>,
 	and will be replaced by the URL.
 
+-flambda-opts <flags>
+         This experimental option will pass specific user flags to the
+        OCaml optimizing compiler. In most cases, this option is used
+        to tweak the flambda backend; we recommend using
+
+        -flambda-opts `-O3 -unbox-closures`
+
+        but of course you are free to try with a different combination
+        of flags. You can read more at
+        https://caml.inria.fr/pub/docs/manual-ocaml/flambda.html
+
+        Note that a regular build with a flambda-enabled compiler
+        requires a large amount of RAM (>= 10GiB) in one particular
+        file, see: https://caml.inria.fr/mantis/view.php?id=7630
+
+        Meanwhile, users with less powerful machines are recommended
+        to disable native compilation with `-native-compiler no`.
+
 5- Still in the root directory, do
 
 	make