Restore documentation of --enable-sjlj-exceptions

[Eric Botcazou]

It had wrongly been put in the Java-specific section of doc/install.texi so 
was removed when the entire section was removed.  Fixed thusly on the mainline 
and 7 branch (and entry moved on the 6 and 5 branches for good measure).

Tested on x86_64-suse-linux, applied on all active branches.


2017-05-29  Eric Botcazou  <ebotca...@adacore.com>

        * doc/install.texi (Options specification): Restore entry of
        --enable-sjlj-exceptions.

-- 
Eric Botcazou
Index: doc/install.texi
===================================================================
--- doc/install.texi	(revision 248552)
+++ doc/install.texi	(working copy)
@@ -1047,6 +1047,11 @@ and for cross builds configured with @op
 More documentation about multiarch can be found at
 @uref{https://wiki.debian.org/Multiarch}.
 
+@item --enable-sjlj-exceptions
+Force use of the @code{setjmp}/@code{longjmp}-based scheme for exceptions.
+@samp{configure} ordinarily picks the correct value based on the platform.
+Only use this option if you are sure you need a different setting.
+
 @item --enable-vtable-verify
 Specify whether to enable or disable the vtable verification feature.
 Enabling this feature causes libstdc++ to be built with its virtual calls