[gimp] plug-ins: fixing automake warnings about source in subdirectories.

[Jehan <jehanp src gnome org>]

commit 883fbd7b65762bb0d2530e995af576478c30c998
Author: Jehan <jehan girinstud io>
Date:   Thu Aug 22 22:28:16 2019 +0200

    plug-ins: fixing automake warnings about source in subdirectories.
    
    This fixes:
    > warning: source file 'images/imagemap-icon-images.c' is in a subdirectory,
    > but option 'subdir-objects' is disabled
    And similar warnings.

 plug-ins/gfig/Makefile.am       | 2 ++
 plug-ins/imagemap/Makefile.am   | 2 ++
 plug-ins/lighting/Makefile.am   | 2 ++
 plug-ins/map-object/Makefile.am | 2 ++
 4 files changed, 8 insertions(+)
---
diff --git a/plug-ins/gfig/Makefile.am b/plug-ins/gfig/Makefile.am
index bdbfe17cab..c0d97e498f 100644
--- a/plug-ins/gfig/Makefile.am
+++ b/plug-ins/gfig/Makefile.am
@@ -1,5 +1,7 @@
 ## Process this file with automake to produce Makefile.in
 
+AUTOMAKE_OPTIONS = subdir-objects
+
 if OS_WIN32
 mwindows = -mwindows
 else
diff --git a/plug-ins/imagemap/Makefile.am b/plug-ins/imagemap/Makefile.am
index 11979109f7..5bfedc2bac 100644
--- a/plug-ins/imagemap/Makefile.am
+++ b/plug-ins/imagemap/Makefile.am
@@ -1,5 +1,7 @@
 ## Process this file with automake to produce Makefile.in
 
+AUTOMAKE_OPTIONS = subdir-objects
+
 if OS_WIN32
 mwindows = -mwindows
 else
diff --git a/plug-ins/lighting/Makefile.am b/plug-ins/lighting/Makefile.am
index 2c89b3ffbf..e5a512aabf 100644
--- a/plug-ins/lighting/Makefile.am
+++ b/plug-ins/lighting/Makefile.am
@@ -1,5 +1,7 @@
 ## Process this file with automake to produce Makefile.in
 
+AUTOMAKE_OPTIONS = subdir-objects
+
 if OS_WIN32
 mwindows = -mwindows
 else
diff --git a/plug-ins/map-object/Makefile.am b/plug-ins/map-object/Makefile.am
index 4d4287a1db..f7c1b5c693 100644
--- a/plug-ins/map-object/Makefile.am
+++ b/plug-ins/map-object/Makefile.am
@@ -1,5 +1,7 @@
 ## Process this file with automake to produce Makefile.in
 
+AUTOMAKE_OPTIONS = subdir-objects
+
 if OS_WIN32
 mwindows = -mwindows
 else