[gimp] INSTALL.in: put libmypaint build instructions back

[Michael Natterer <mitch src gnome org>]

commit ac8c130719e8652ea687afbd3240290afc0bc118
Author: Michael Natterer <mitch gimp org>
Date:   Mon Dec 28 21:18:52 2015 +0100

    INSTALL.in: put libmypaint build instructions back

 INSTALL.in |    7 ++++++-
 1 files changed, 6 insertions(+), 1 deletions(-)
---
diff --git a/INSTALL.in b/INSTALL.in
index abb1493..c0122b4 100644
--- a/INSTALL.in
+++ b/INSTALL.in
@@ -81,7 +81,12 @@ header files installed.
 
  10. For MyPaint brushes, brushlib (libmypaint) @LIBMYPAINT_REQUIRED_VERSION@ is used.
 
-     Brushlib repository is hosted at:
+     To build and install libmypaint:
+
+       `scons prefix=<your prefix>`
+       `scons prefix=<your prefix> install`
+
+     The libmypaint repository is hosted at:
 
         https://github.com/mypaint/libmypaint