[gimp] INSTALL: add some information about PKG_CONFIG_PATH for mypaint-brushes.

[Jehan Pagès <jehanp src gnome org>]

commit 0f29e67078bff2e70c110f62944de315fbce0589
Author: Jehan <jehan girinstud io>
Date:   Wed Jan 3 19:20:29 2018 +0100

    INSTALL: add some information about PKG_CONFIG_PATH for mypaint-brushes.
    
    I didn't think I'd have to be this detailed, but it appears a lot of
    people were not aware that data package .pc files would end up under
    share/pkgconfig/ (and not lib/) and have problems with installing this
    new "mypaint-brushes" dependency. Hopefully this will be enough.

 INSTALL.in |    6 ++++++
 1 files changed, 6 insertions(+), 0 deletions(-)
---
diff --git a/INSTALL.in b/INSTALL.in
index 7a44571..8b8ad19 100644
--- a/INSTALL.in
+++ b/INSTALL.in
@@ -93,6 +93,12 @@ header files installed.
      particular tag "v1.3.0". In particular do not install from master
      which installs brushes incompatible with GIMP.
 
+     Also this is a data packages and therefore it will install the
+     pkg-config file inside `$PREFIX/share/pkgconfig/`. If you install
+     mypaint-brushes from repository in a non-standard prefix, you will
+     have to make sure your $PKG_CONFIG_PATH environment variable also
+     lists `$PREFIX/share/pkgconfig/`.
+
  11. You may want to install other third party libraries or programs
      that are needed for some of the available plug-ins. We recommend
      to check that the following libraries are installed: libjasper,