[gimp] INSTALL: update a bit the installation instructions.

[Jehan <jehanp src gnome org>]

commit 15c1e04c27b87ef5633837e09d32c05a542b5947
Author: Jehan <jehan girinstud io>
Date:   Sun Dec 20 04:00:06 2020 +0100

    INSTALL: update a bit the installation instructions.

 INSTALL.in | 35 +++++++++++++++++++++++------------
 1 file changed, 23 insertions(+), 12 deletions(-)
---
diff --git a/INSTALL.in b/INSTALL.in
index b0064c872a..e692ef08b2 100644
--- a/INSTALL.in
+++ b/INSTALL.in
@@ -96,19 +96,24 @@ header files installed.
         https://github.com/mypaint/libmypaint
 
      If installing from repository, do not install the master branch!
-     Checkout the tag "v1.3.0" instead, or simply install from a tarball
-     or from your favorite package manager.
-     You may also checkout the "libmypaint-v1" branch, which is the
-     development branch for libmypaint v1.x and has some more recent
-     fixes.
+     Checkout the last tag "v1.y.z" from `libmypaint-v1` branch instead
+     (for instance "v1.6.1" tag at time of writing), or simply install
+     from a tarball or from your favorite package manager.
+
+     In particular, do NOT install tags or release tarballs versionned
+     "v2.y.z". GIMP depends on the version 1 of libmypaint and is
+     incompatible with the version 2 (which is still experimental anyway
+     and has no stable release at time of writing).
 
  11. We also need the mypaint-brushes data package:
 
         https://github.com/mypaint/mypaint-brushes
 
      If installing from repository, install from branch "v1.3.x" or the
-     particular tag "v1.3.0". In particular do not install from master
-     which installs brushes incompatible with GIMP.
+     last tag "v1.y.z" (e.g. "v1.3.1" at time of writing).
+     In particular do NOT install from `master` branch which installs
+     brushes incompatible with GIMP (the `master` branch brushes are
+     targetted to libmypaint upcoming v2, not v1).
 
      Also this is a data packages and therefore it will install the
      pkg-config file inside `$PREFIX/share/pkgconfig/`. If you install
@@ -132,8 +137,8 @@ header files installed.
      the plug-in is mostly useless.
 
  14. GObject Introspection requires the following dependencies to be
-     built and installed with introspection as well: cairo, GdkPixbuf,
-     GEGL, GIO, GLib, GObject and GTK+.
+     built and installed with introspection as well: babl, cairo,
+     GdkPixbuf, GEGL, GIO, GLib, GObject and GTK+.
 
  15. Windows builds can now generate backtrace logs upon a crash.
      The logs will be available in: %APPDATA%\GIMP\@GIMP_APP_VERSION@\CrashLog\
@@ -215,10 +220,8 @@ header files installed.
      Python @PYTHON3_REQUIRED_VERSION@ and PyGObject for Python 3 plug-ins
      GJS for Javascript plug-ins
      LuaJIT and LGI for Lua plug-ins
+     dot for "Show Image Graph" (unstable branches)
 
-Please make sure you don't have any old GTK+-2.x, jpeg, etc. libraries
-lying around on your system, otherwise configure may fail to find the
-new ones.
 
 
 Generic instructions for configuring and compiling auto-configured
@@ -233,6 +236,14 @@ compilation and installation output is not shown.
   % make install                  # install GIMP
 
 
+Note: if building from repository (not tarballs), you may notice a meson
+build system is also available in the repository. It is not the main
+build system and is not recommended for stable packages as it still has
+several known issues, including build flag errors leading to crashes and
+missing features.
+We only invite contributors to use the meson build if they wish to help
+with debugging.
+
 The `configure' script examines your system, and adapts GIMP to run on
 it. The script has many options, some of which are described in the
 generic instructions included at the end of this file. All of the