[gimp] INSTALL: add description of --enable-vector-icons.

[Jehan Pagès <jehanp src gnome org>]

commit 28942c984373a9d604d47e7b34882ff9ccb24110
Author: Jehan <jehan girinstud io>
Date:   Thu Jun 2 01:16:17 2016 +0200

    INSTALL: add description of --enable-vector-icons.

 INSTALL.in |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)
---
diff --git a/INSTALL.in b/INSTALL.in
index 57ffb50..17dadf1 100644
--- a/INSTALL.in
+++ b/INSTALL.in
@@ -189,6 +189,9 @@ options can be listed using the command `./configure --help'. There
 are several special options the GIMP `configure' script recognizes.
 These are:
 
+  --enable-vector-icons.  This option installs vectorial icons instead
+    of raster icons.
+
   --enable-shared and --disable-shared.  This option affects whether
      shared libraries will be built or not. Shared libraries provide
      for much smaller executables. The default is to enable shared