[gimp] Issue 5870: add reference to GNU documentation in INSTALL.

[Jehan <jehanp src gnome org>]

commit 60f1095d4b5ef4c187adbdb0ebe07f359301404d
Author: Jehan <jehan girinstud io>
Date:   Fri Nov 6 15:52:57 2020 +0100

    Issue 5870: add reference to GNU documentation in INSTALL.
    
    We are not going to duplicate the whole autotools documentation. Let's
    rather just state we follow the GNU build standards, hence refer to
    relevant documentation with a link.

 INSTALL.in | 11 +++++++++++
 1 file changed, 11 insertions(+)
---
diff --git a/INSTALL.in b/INSTALL.in
index 7a0d1e7fee..830037d0b6 100644
--- a/INSTALL.in
+++ b/INSTALL.in
@@ -551,3 +551,14 @@ Makefile(s), the header file containing system-dependent definitions
 The file `configure.ac' is used as a template to create `configure' by
 a program called `autoconf'.  You will only need it if you want to
 regenerate `configure' using a newer version of `autoconf'.
+
+More autotools commands
+=======================
+
+GIMP uses the autotools build system which follows the GNU coding
+standards. In particular all usual standard targets are obviously
+supported (e.g. `make uninstall` for uninstalling) as well as common
+variables and options, cross-compilation host and target, etc.
+
+If you wish to know more, please refer to GNU documentation:
+https://www.gnu.org/prep/standards/html_node/Managing-Releases.html