[gimp] build: run flatpak-builder with --ccache option for faster builds.

[Jehan Pagès <jehanp src gnome org>]

commit e545116b89817b7cb033fe661d94fa05c63ef12f
Author: Jehan <jehan girinstud io>
Date:   Thu Oct 5 00:16:34 2017 +0200

    build: run flatpak-builder with --ccache option for faster builds.
    
    So I discover today that there is an option --ccache to request
    flatpak-builder to compile using ccache, which is obviously a great idea
    when rebuilding the same deps too often. Update the howto with the info.

 build/flatpak/flatpak-howto.txt |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/build/flatpak/flatpak-howto.txt b/build/flatpak/flatpak-howto.txt
index e38cfd1..8b73fd5 100644
--- a/build/flatpak/flatpak-howto.txt
+++ b/build/flatpak/flatpak-howto.txt
@@ -88,8 +88,8 @@ Or if you already have a build:
 
 * Run flatpak builder with the following commands:
 
-> flatpak-builder --arch=x86_64 gimp-flatpak-build-x86-64 org.gimp.GIMP.json
-> flatpak-builder --arch=i386 gimp-flatpak-build-i386 org.gimp.GIMP.json
+> flatpak-builder --ccache --arch=x86_64 gimp-flatpak-build-x86-64 org.gimp.GIMP.json
+> flatpak-builder --ccache --arch=i386 gimp-flatpak-build-i386 org.gimp.GIMP.json
 
   Note: if you ever want to check the `config.log` of any of the module which
   failed to compile, it is available in a hidden directory of your working path.